No, it shouldn't be allowed, because the last thing you want to do is to open the door of people arguing over which changes should be allowed and which shouldn't. You have a perfectly good speedgame here that works great the way it is, and the ONLY thing this line of discussion would result in is a massive shitstorm that won't benefit anyone or push the game forward.
Sprites aren't cut off to match hitboxes. Sprites have a little padding (used in 3D texturing). A 16x16 sprite might look 18x18 in the game data, although it will still appear 16x16 in-game since the remaining pixels are padding.
I'm just warning you that if you decide to allow X modification to the game, later down the line you're gonna get real sick of people complaining that Y and Z modifications aren't allowed, and that the rules and game suck.
