In response to Lee "Q-Tip" Metcalfe regarding the subject:

Since I fly an early 235 with the Lyc O-235-L2C with the Lycoming hi comp.
pistons, my experience will not necessarily be germane to your O-320 and is
merely anecdotal. But FWIW, I've had no problems in the 500+ hours since I
changed out the original pistons. I've seen very little change in
temperatures in all flight regimes, however that may be because I over-did
the cooling system and have always had temps that were actually somewhat too
low (except for unusually long ground hold times in the summer).

According to the Lycoming data, these pistons raise the engine's HP about 7
to 8%, e.g., from 118 continuous to about 125. Unfortunately, I can't say
what, if any, performance improvement was gained since I also went to a MT
C/S prop at the same time.

My only concern at this time is the bruhaha over the availability of 100LL
in the future since I'm sure the higher compression would not take kindly to
any lower octane fuel. I have no idea what the detonation margin for my
engine is now with 100LL but I'm sure it would be lower (and perhaps
problematic) with unleaded or otherwise lower octane fuel.

Cheers,

Dan Schaefer