The vernier pulley allows you to adjust the timing of the exhaust camshaft in relation to the rest of the engine. It's a mod that typically gives good results, a 10bhp increase is not unheard of, but it's quite an involved mod to really get the best out of it. You need to remove the standard cam pulley which means removing the timing belt (so you might as well put a new one on while you're doing the job) and then the car really needs an hour on a dyno with a competent operator who can tweak the timing of the cam to give you the best gains.

There are people who will tell you just to retardt he cam by 6 degrees and that will guarantee you 10bhp, but every engine is slightly different so the actual change in the timing and the results you will get will be different from engine to engine - hence the need to use a dyno to get the best out of it.

A worthwhile mod if you do the job properly, but nowhere as simple as just fitting the new pulley.

ETA - If you're definitely doing the vernier AND getting the car remapped then do the vernier first.