Aluminum is just fine for mountain bikes. It's relatively cheap and the stiffness is not a detriment to the ride comfort due to the large tires and the suspension. It doesn't rust. There has to be an economic or performance benefit to another material and I don't see any for SS in an ATB .
Lower center of gravity generally means low ground clearance. It's not all that productive to hit your pedal on rocks and roots, so there is a lower limit. My 2003Speciaized FSR, designed for XC is about as low as I'd want to go.
Center of gravity is generally set by the bottom bracket height as when one is riding the rough stuff, he's on the pedals and most of the rider's weight is on the bottom bracket. The rider is "live weight" meaning the riders center of gravity moves due to the body English used to maneuver and stabilizes the bike in the rough stuff, turns and in the air. The height of the center of gravity is not all that important. it's more a function of how the rider moves his CG than the actual height.
My FSR is much faster in the turns, yet I sit higher than my old hardtail. It also has two positions for the lower shock bolt. By moving the the shock to the more rearward bolt position, I reduce the travel and the initial height by 15mm.
Al