Andy, see my thread where I consider and do exactly this. . .
http://forums.pelicanparts.com/showthread.php?t=368320
69S pistons are the same as 906 but they dont come in 81mm I don't believe. I do think Mahle 81 are available as 10,3:1.
Here's the problem: our small bore and deep combustion chamber make it difficult to get high compression without detonation on single plug. In order to get 9,8:1 or higher the dome of the piston has to be pretty big, 38 cc or more. This results in comparatively poor flame propogation around the "bump."
In the end I settled on 9,5:1 as a compromise between the original and the S, something that I could run on 93 R+M/2 "gasoline." Also, this car will be operated during high ambient temperatures in the summertime, so that's definitely a factor in the compression and octane decision.
With regard to cooling this is something I also considered and decided NOT to build the engine larger due to the need for external cooling. I had the oil cleaner ultrasonically cleaned and will use all new hoses, and have a fan with a 1,8:1 ratio center (not for concours but I have another original), I suppose you could also use a later style fiberglass shroud with the channel to feed more air to the oil cooler. Also, my case has piston squirters and the oil bypass mod, both of which help with cooling.
Hope this helps!