how much HP??
 

Does it add when shaving the fuel plate to 0??

Yes it does, how much I'm not sure. It mostly depends on afc tuning, what rpm you run, if your running stock Psi on the turbo or not. If you running a stock truck like a 165horse truck don't expect to make much more then 220 to the wheels with just shaving the plate and maybe tuning. You will need a 3-4 gsk and a boost elbow minimum. Shaving the plate, and learning to tune the afc is a good place to start though.

With a '98, it should be a 180 truck at least.

Depending on the AFC tuning, a zero plate could be good for as much as 100hp or more. But don't expect to feel a third of it getting to the ground if you have a stock auto. Or for that auto to last very long.