Weighted Megas might fare a little better, but I understand that they have weird aerodynamics for one reason or another, so I'm not sure.
Regarding a stock experiment, your Megas should be more stable, or so my own (highly basic) testing shows me. 200 feet isn't honestly that far anymore, not in the big scheme of things; even some of Xplorer's kits are hitting that range without any further upgrades.
Nonetheless, if you want it to go at 250+ FPS, Micros might be your best bet, due to having less drag. Whatever you do, I'd advise just reinforcing the living hell out of a Titan tank, pressurising it as much as physically possible, and then firing it. You'd do well to find a way of doing it from twenty feet away, though - that, or find some body armour to wear while you do it (see if the Paintball club will let you borrow their stuff ).