If you want a super high velocity round, those exist as specialty varmint rounds. They drive small caliber, lightweight bullets at velocities up to around 4,000 fps. You still need sectional density to maintain momentum and deliver energy downrange, though, so a short, fat, light bullet doesn't work. In other words, you're better off with a tiny caliber (17-22) bullet made of lead than a large caliber bullet made of a lightweight material.
Incidentally, 4,000 fps is about the theoretical and practical maximum for any projectile using nitro-based propellants. The reason has to do with the rate at which the combustion gases expand. Once you get the projectile moving as fast as the combustion gases, you can't accelerate it any faster. In addition, barrel erosion because a huge problem as you reach toward those maximum velocities.
The bottom line is that there is no need to play with large caliber bullets filled with a light material like aluminum, or even hollow. If your objective is to drive a projectile extremely fast, then the way to do it within the bounds of existing available firearms technology is the modern varmint rifle driving a small caliber projectile.