if you have the money, I might go for a disc up front. But you'll need a new fork and brake.
Echo Urbans are great forks for 20".
You can also go second-hand and the quality will be pretty much the same, just with paint scratches.
I have a Hope Mono Mini and that works great, but pretty much any trials-spec disc brake will work for a 20" because there's less leverage needed to make the wheel stop turning.
EDIT: Answering your actual question, grinding your front rim will make your bite and hold stronger, because it adds more friction. However, it means that if you're going down a hill and you need to stop, you'll be forever making lots of noise.