I haven't worked on a "H" for a while, and I know the valve train geometry was it's biggest flaw, but I don't believe high compression/heat helped it either. I guess I've built/maintained a few engines, and high compression always makes for high maintenance, and a lot less reliability. It only makes sense, your putting more load on the parts, so things are going to break more often. These engines were designed with a large safety margin, and doen't get me wrong, I'm glad there are guys out there who are willing to push the limits. I doen't fly alone however, and I feel a responseability to whoever is in the other seat to put the safest, most reliable engine under the cowl that I can. Be careful getting advise from these engine hop-up shops, they are in the business of selling you their product or service, think they are going to tell you anything bad? I was hired to salvage one of these hopped up, high compression, "doen't worry, it can take it" engines from one of these companies, (I won't mention which one). It grenaded with very little time on it, it was really impressive. The biggest recognizable piece was a small section of the case that managed to hold 2 cylinders together. I've never seen a Lycoming come apart like that, I didn't salvage much.
PS on cylinders,
I've built up engines with Milleniums, and the owners swear they have more power, no dyno tests or anything, this is just the feedback I get. I have had very good luck with them. I like the extra metal they put in the exhaust ports, right where the stock ones always crack.