wow what broke on them ? was it the piston or rod ? .. When using a modded engine you need to use a different running procedure as they are high performance machine, they need different care procedure, most important long warm up periods.. and cool down 's as well..

Reducung clearance dosent reduce mileage. and detonation either premature or latent can be fixed with using the right plug. you can use 30% nitro , with the copper shim out and a medium plug with no hassles, you will get pre detonation if the sleeve is modded in which case you use a hotter plug.

Modding engines is a personal preferance, If done correctly will run better than a stock engine, better power, fuel economy and temps..