I htough you might ask that
and I didn't have time to write an answer. Not a simple concpet but I'll try to keep it so:
1. Bumpsteer is toe change that is caused under suspension compression.
2. You get this toe change becuase your toe is set when the lower arm is static, not moving.
3. Even in a standard car the the lower arm and steering rod follow a slightly different arc as they are pivoted from a different position.
4. As the arms move, they follow completely different arcs so you toe out under compression as their relative lengths change. (See point 2)
5. If you over lower your steering arm, you can still get excessive bumpsteer, you'll just be on a different part of the curve. If you loook at this (After a quick image search)
Our suspension is like Example 2 (but worse as our arms are different lenghts too!), Nissan have put a lot of time into making the 2 arcs as close as possible with the bits they have in their parts bin. Lowering the car will only make it worse.
6. Given what we've got, you can't eliminate bumpsteer, you move your steering arm pivots so its arc of travel relative to the lower arm are as close as possible and the distance change between the two arcs is minimised. The only way to eliminate it is possible is if the axis are in the same horizontal and vertical location.
7. Unlike camber and toe, measuring bumpsteer needs professional equipment or is a complete pain as you need to take 10 measurments across the range of movement of compression and see how much toe changes.
8. Example of the extremes the SR20 race boys are going to: http://www.sr20forum.com/showthread.php?t=91930&page=3
9. If you did lower your car extermely for race conditions, you can correct the bumpsteer, unfortunatly this won't correct camber change you will also get. These guys are raising the lower ball joint relative to the lower arm to compensate and are (still) looking into manufactuing their own lower arms!
10. If you can understand what they are saying, some of these guys are professionals and have a lot of knowledge and experience with our cars. One of them has written many good articles in SCC using our suspension as an example.
Hope that helps from an informed amateur.