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.
To address Twitch's new 100-hour highlight limit, we are making a few changes to make sure that the highlight limit (as well as any future Twitch decisions) doesn't affect the leaderboard's integrity. Effective immediately, we are no longer accepting runs submitted through Twitch highlights, reg