[skim]

Well, GT1 block, which is shared between 997.1 TT and GT3/RS, and 997.2 GT3/RS (I'm told 997.2 TT went with the A91 engine which is shared with all other 911s) don't suffer from RMS. I don't think RMS kills the engine necessarily right away, it's just if you don't fix it soon, your engine will eventually die from from oil starvation.
I was also told while shopping for a 993 that 993 has RMS issue as well although we know it's all air-cooled pre-996. But didn't bother looking up because I picked up e86M.

Just my opinion tho, although cheaper (and possibly could be more fun) than 911, in the long term cayman/boxster ownership cost will catch up to 911 so I think it's better to just get a 911 from beginning. Again.. I'm just speaking from what I'd do.
GL