The upgrade to WPC style flipper mechs on a system 11 is replacing the standard conical spring that sits around the plunger and pushes the the flipper back to rest when not activated, with the WPC style return spring (and associated arm extension on the flipper pawl) that pulls the flipper back to rest after flipping.
The WPC style feels snappier because it doesn't induce any drag on the coil plunger, and especially over time since it doesn't wear into the plunger or link.
Upgrading to WPC style is purely mechanical, and in no way effects the electronics of the game (assuming the flipper pawl extension still makes appropriate contact with the eos switch)
I just did this to a Taxi I picked up a little while ago. Simple and effective. Literally changing the spring style and adding the arm to hang it from.