I have a cure 3 bolt on mine and there's isn't too much difference there's a few simple upgrades like a cure 3 bolt, hush bolt, and a zick kit. It's a great marker and performs well as standard so consider keeping it that way!
I have the Cure 4 Bolt and a Violent Series Trigger.
Both are pretty good. I thought about get the APE board to add the screen, but the grips look awful!