From my push bike days messing around with gears and wheels etc the 15/48 tooth combo should give you alot more get up and go, 0-60mph times will drop but all the thing will probably want to do is wheely. Also every gear will feel shorter so there for insteed of being able to do say 20mph in first you will only be able to 15mph or so.

The only big draw back to this mod is that your top speed, or comfy cruising speed will drop, by how much dont know.

__________________
Plough on