In some kind of order of importance:
- New trigger must be top of the list, the standard trigger is uncompromisingly awful.
- Next thing you would want to do is get a delrin bolt, and probably a stainless hammer to balance the weight saved by the lighter bolt. This reduces kick and makes the whole thing run faster.
- You ought to add a tapeworm or full-flow modification, depending on which version body you have.
- The stock barrel isn't great, you could replace it with any of the aftermarket barrels around.
At this point you should have a gun that rips, but if you want to go further:
- If you want it even faster, replace the circuit board with a WAS Equalizer.
- Lastly, you could add an LPR for slightly better efficiency and consistency, but it's a lot of money for not a huge improvement.
New Designz make many of the best upgrades for the Impulse, but at one point there were dozens of companies all making parts. Probably not quite so widespread now, but you still shouldn't have much trouble finding them.