Kahloq,
What battery packs were you using that sagged, and how old/cycles wee they?
i've converted all my larger planes to glow/gas now, lost several on battery power, problem is when your glow/gas plane goes in you usually don't damage your motor, with ALL my 'sparkie' crashes either or both motor and batteries were destroyed[&o] makes for a very expensive crash!!! I wouldn't be concerned with the power/speed of the gas motors, it's easy to use a larger pitch prop to get the scale speed you want.

Cheers, Paul.