The BMW 1M is basically a 135 with some tweaks. The Cayman R is basically a Cayman S with some tweaks. Both appear to perform much better than their base cars. If you look at lap times, you would conclude they are radically better. But that is a bit misleading.
I believe the tweaks to the 1M and the R are pretty similar, and both are what enthusiasts do to the base cars.
Neither one really has a modified engine at all. That is mildly disappointing. The Cayman R gets +10 hp from air flow mods (which enthusiast owners do to their Cayman S), and the 1M gets +30 hp from air & ecu mods (enthusiast 135 owners do air & ecu for +any hp, 30 is no problem). Also, neither one is really lighter. If you weigh them in the no-radio, no-AC, lightweight seat spec then they seem a bit lighter, but of course you could do that to your S/135 if you wanted. You can rip out your AC and put Recaros in your current car, that's not a serious way to lighten a car, so they are a big fail in that respect (the actual structural weight savings on both cars is something like 30 pounds; it's also retarded how they do the lightening in these cars; they remove your AC and radio, which you want, but then they still put a cosmetic plastic piece over your engine; WTF, remove the useless cosmetic weight and leave the functional shit please; if you want to get serious about lightening, I could go for manual windows and manual door locks and trunk release and all that, you can get rid of all those motors).
(the lack of engine mod is particlarly disappointing in the Cayman R, since Porsche has got a bunch of engines just sitting around that could have gone in there; they could have put the 3.6 version of the 9A1 in there instead of the 3.4 that the Cayman usually gets)
What has been done? Well the formula is exactly the same for both cars : LSD & Stiffer rear.
Porsches and BMW's have been set up for horrible understeer for the past many years. You may have seen me write about this exact issue with Porsches before. Most of the cars don't come with LSD; on Porsches you can option an LSD, but that is sort of a kludge because they aren't properly set up (you want different suspension setups for cars with LSD vs. not). Basically all they're doing with the R and the 1M is undoing the bad setup of the S and the 135. Particularly I think with the 135, it's a travesty that such a good engine has been sold with fucking run-flat tires and no LSD. So they're just fixing that sin.
Once you look at the 1M and R not as new special sporty models, but just as fixes to make these good cars the way they always should have been, you see the point of them. Anyway, some details :
BMW 1M : N54 +30 hp from piston rings, air, ecu LSD (viscous) proper rubber transmission oil cooler M3 rear suspension and rear subframe (stiffer) wider track light flywheel steering from E3 ecu (throttle response?) lots of actual M3 parts Cayman R 9A1 3.4L +10 hp (exhaust manifold, ecu, 7400 vs 7200 rev limit) LSD (friction clutch pack) lower / non-PASM / stiffer suspension more negative camber (still not enough) stiffer rear sway not actually GT3 parts (not adjustable)
The differences between the 1M and 135 are a lot more significant than the differences between the Cayman R and S. In both cases the price premium ($5-10k or so) is so small that of course you should go ahead and buy the 1M/R.
I know a lot more about Porsches than I do about BMW's, and I can say that as usual Porsche have cheaped out and done the absolute minimum to hit their performance target. They could have easily grabbed the GT3 suspension bits, which is what you really want because they are adjustable in a wide range from good on the street to good with R-comp on the track, but no, that would have cut into their massive profit margin, so instead they give you new non-adjustable suspension bits, that are better than the S, but still not actually good enough for the serious enthusiast. You can just see it in every little aspect of modern Porsches; they don't give you the good brake cooling ducts, they don't give you the good shift cables, they don't give you the good throttle body, etc.
To be redundant, basically what a serious Cayman S owner does is they buy the GT3 front LCA's, GT3 sway bars, maybe a stiffer spring set, and a good aftermarket LSD. All that costs you a little bit more than the Cayman R, but then you have a much better car. So if you're going to track mods for your car anyway, there's no point in starting with the R, because you're going to replace all it's parts anyway, just start with the S.
It's sort of an odd fact that all modern Porsches (below the GT3) are shipped intentionally crippled. This not only makes them look back in magazine tests, it makes them look bad in comparisons like "fastestlaps.com" that compare stock cars, and it is a real problem for people who want to race them in SCAA "stock" classes. It's strange and very annoying. It's very useful when a car manufacturer offers a "super sport" option pack (such as the Mustang Boss 302 Laguna Seca) - even if you don't buy that car, it gives you a set of example parts to copy, it shows you how to set up the car for maximum performance, and it gives the magazines a way to test the car in properly set up trim. And if it's an option pack (as opposed to a nominally different model like the R), then you're allowed to use in SCCA stock racing.
One misgiving I have about the Cayman R that would give me pause before buying it new without driving is that it is lower and stiffer than the Cayman S, which is already quite low and quite stiff. It's sort of cheating to make a car that handles better by making it lower and stiffer. It's like making a compressor that gets better ratio by using more memory. It's not actually an improvement in the fundamentals, it's just dialing your trade-off slightly differently on the curve. The really impressive thing is to make a car that handles better without being lower or stiffer.
(the other problem with buying the Cayman R new is that Porsches are absurdly over-priced new, it's basically a $50k car being sold for $70k just because there are enough suckers and no competition)
Attempt to summarize my usual descent into rambling :
The Cayman R is a nice thing because it shows everyone what a properly set up Cayman S can do; the Cayman S has looked falsely bad in lots of tests (see for example Fifth Gear test where Tiff complains of understeer) because of the way it's sold set up all wrong, so the R is good for magazines, but it really isn't that special of a model, and if you wait until 2014 for the next gen 991 Cayman it will be better.
The 1M on the other hand is a very special model that will only be around this year, and gives you loads of goodies for the value; if you were considering a small BMW, the 1M is clearly the one to buy.