Alot of guys run the 2.93 with no or just the lower 2 100mm idlers and retain there covers. They have little or no slip. The upper idler does help though. Of coarse it depends on the pulley you use too.
If you go with 2 100mm idlers to take the place of your 2 90mm lower idlers you will do ok. You might want to check into that new blower pulley with the cross cuts and holes in it. I hear they work pretty good. Then you can keep everything under the factory belt cover.
I wish I kept my blower cover on too man.. You start to realize all the things you are missing once you have it off. I was at the dyno and I was icing the car down and I wanted to keep the ice on the supercharger while I was dynoing it.. Well, no dice.. We'd have some air borne ice bags and an ugly mess if they ever slid off the blower onto the overspinning pulley. Heh.. that would definately not be cool. Just tell people that you took the blower cover off for weight issues heh.