If you enjoy the game, it wouldn't hurt to try. You say all of them seem to take way too long. You can start out small, and run single cups(check misc. in the categories tab) to get yourself used to using the timer as well as get a taste of what an mkw speedrun would look like.

I am not a mod, but in the past, I have used a shield upgrade in some of my single cup runs and they have been accepted. The single cup, the only goal is to get the fastest time. So upgrading your shield just ups the chance you are going to get one in a run. Even then, it's not like you will get one every run. So I would say go for the upgrade, and may I see some of your times on the leader board. Godspeed!