The most bang for your buck will be to start out with deltas, by far. Any half decent .45 with a 8x8 or 9x8 prop will give you the kind of performance that will put a big smile on your face [if not make you laugh out loud].
Don't look for any engine mods to make a huge difference in air speed. You might hear a rpm difference, but the difference in speed won't be as noticeable. After market exhausts might provide 5 mph for $50-$100. Save that money and buy glow plugs and props.
If you ask me, the Q-500 planes don't seem fast unless they're being towed by a Nelson or a hot Jett.
It's the .40 sized delta that is the best first step for cheap speed.