Without knowing the exact steel plate mounts you are using now it's hard to give definitive answers. If the mounts you have now are like the Planted E46 bracket then you will probably be able to lower the seat a little and maybe drop a pound or two. The aluminum bases will also be stiffer because they bolt directly to the chassis without the little flexible tabs like on most of the steel designs I've seen.
With the aluminum bases, no slider, and a good set of side mounting plates you can put a fixed back racing seat nearly on the floor of an E46. With all the adjustability this type of setup offers you can really dial it in to your comfort.