Here's a pic of the pre-revup plenum and revup plenum. Shaving the pre-revup snouts basicaly turns it into a MREV2 and that's the only difference.
For the cost against gains for the pre-revup I'd say it's not worth it. But being boosted it maybe beneficial.
Best thing to do is get yourself a pre-revup lower collector (save using your own) and I'm sure there's a guide on my350 with how to shave the snouts, which will also tell you how much to take off.
On the revup engine, the plenum spacer basically does sod all, but match it with the MREV2 and you get very good gains.