Hi Frank;
I don't have any experience with the ST G20/23 so can't comment on that one, but the Fox .25 should be a decent choice for the performance you say you are looking for. Just use proper fuel, and prop it correctly, and you'll get a nice sport model, I would guess about on par with what the OS LA.25 would perform at. Try some 9-5 to 9-6 props to start of with, after the engine is broken in properly. You also didn't specify if it was a kit/scratch built Streak, or an ARF. The ARF is quite light, but a kit built version should fly sedately with the Fox .25 also.
Good luck and have fun,
Dan McEntee