Ok, I'll have a go at comparing the two...
I drove the Cayman R last week on an OPC test drive, drove it for about 30 minutes on motorway, a-roads and b roads so had a reasonable amount of time to try it.
Steering - it has a lighter feel (mostly due to the lighter wheels I'd guess) it makes it feel more GT3 like, when you push the car into corners the response and feel through the wheel is typical Porsche, slightly better than my S which can feel heavier and less fluid if that makes sense.
Suspension - without PASM the car reminded me of my old GT2, a more natural feel to the suspension, you realise how much PASM does when you drive a car that doesn't have it. The ride is somewhere between normal and sport settings on PASM, it'd be easy to live with and plenty firm enough for track use, it's a great setup IMO.
Power - to be perfectly honest I struggled to notice any increase in performance, I had my foot flat to the floor a couple of times and I'd be surprised if my car lost any ground on it. I've heard lots of people say that the new DFI is more eager to rev, I didn't notice that, it felt just as lazy below 4k rpm and just as lively above it as my non-DFI car. What I did notice is that the power delivery and engine note is much smoother. The noise from my engine sometimes sounds like it's about to chew itself up but this had an overall smoother tone.
I was hoping to notice a bit more from the limited slip diff, I pushed the car into some roundabouts and tried to get on the power early to see what happened and it didn't feel like it was doing much, it didn't feel much different to my car in this respect. I've driven GT2s/3s and the diff has always been audible and noticeable at slow speeds, I didn't get that from the Cayman R, I'd guess it's setup to allow more slip than the GTs? I'm no expert driver and someone who knows what they're on about would no doubt 'feel' the diff but I didn't notice it handling much differently to my car on the test drive.
The interior upgrades like PCM3, bluetooth, ipod connection etc are great, they lift the car a bit and make it feel more up to date. The door pulls are good fun, the new design steering wheel is fantastic, slightly thicker and softer alcantara than mine, very nice. The car I drove had a vinyl dash and it felt cheap compared to the full leather in my car. I wasn't bowled over by the wing on the back of it, the angle of the hatch on the Cayman doesn't lend itself to a 911 type spoiler, the R would look better without it IMO.
All in all it's a brilliant car and is probably about 10% better than mine in most areas. The cost to upgrade was around £30,000 on a new R. That's where the problem is for me, a second hand Cayman R at 40k is a good car for the money, at £60k it's not worth it and it's certainly not worth £30,000 more than my car.
I was having a look around the 997 GT3 at Byfleet yesterday, for 60k that's a lot more car, one that I might be going for another look at