All else being equal there's no difference in HP between a ringed and ABC engine. The apparent difference in compression is only when you turn them over slowly (as when starting) where's there's a bit of time for some pressure to leak past the ring gap. Once they're running there's no time to leak. AFAIK only OS offer a choice of ringed or ABn but even with their engines the ringed is reserved for their helicopter engines which actually operate normally at higher revs than used in planes (rings are more forgiving of dirt ingestion). Ringed engines though are limited to around the 20,000 area where the ring can start to flutter, or vibrate, in the ring groove.

HP figures are just for advertising to make you think brand A is better than brand B. Jett are more honest about it and simply say that if you use x% nitro and size/brand prop then you'll get 17,500 revs or whatever.

In my book, the ST G51 is a very good engine but it's ringed so takes some time to run in completely. If it ever wears out after your kids have grown up then you just buy a $10 ring and wait until the grandkids have grown up .