I'm looking to do the new SL55 pierburgh pump 7.00228.51.0 but was simply going to hook it up to the OEM filter (Mann WK-720) I bought from from autohaus.

Okay- so the WK-720 has an internal pressure regulator set to a specific value....(anyone know what value?)
So I hit it with some more grunt from the SL55 pump and .... I'm guessing as any pressure regulator would - it will simply burn off any excess and run it back to the tank.

If the demand arises (WOT & 21psi) there will be plenty of backup to keep the regulator in the filter from running out of puff.

Is this understanding sound?

and

If the regualtor is passing all that excess back to the tank - I'm guessing she's pulling some serious amps through the whole rpm range (spec sheet for 7.00228.51.0 says 16Amps at 300lph).
I cant recall what the stock one pulls but probably something around the 6-8Amps mark.
I'm guessing....check the wiring and fuse is okay hey.
I do recall somewhere someone had ran an extra wire...but not sure where I read it (could have been SL55 forum).