I use a G6 on a standard 55gal, 48"x13"x20 and I chose to have a seperate powerhead because of the length. Now Pugman uses two on a 180P thats working well, so one G6 could be perfect for the 90P. The lily pipe could have something to do with it seeming to not have as much flow because they are designed to spread the flow out. Pugman is also using pendants with his setup, there are a few similarities between your two tanks, if you havent seen it, here is his thread.
http://www.plantedtank.net/forums/ta...80p-build.html