I get significantly better MPG with the high compression engine (and tight double quench pads and more airflow) than the stock one, it costs about the same per mile and is more fun to drive, quite a bit further range, more power everywhere in the RPM range, a lot less downshifting to maintain speed up hills, very snappy reving.... I have not found a downside since I had the heads laying around and the engine apart anyhow. If your really concerned about the compression ratio you could run a 6.1 cam to bleed a little cylinder pressure or you can run the thick head gaskets like everyone else does but the reduced quench is gonna make it even more detonation prone.