ABS recalibration
 

I'm starting to get a little worried that changing the weight distribution of my car will mess with the ABS system. I know that it monitors the individual wheel speeds, and reacts based on the slippage, but how much does the weight distribution come into play? Also, if it makes a significant difference, can the control unit be recalibrated, or would I need to get a dedicated racing controller?

Weight distribution should not have any impact. The thing to be careful with is tire diameter - don't stagger them too much front to rear. I'm not sure what the actual tolerable difference is, but I try to keep mine within 0.5" of each other.

0.5" diameter or circumfrence?

The rule of thumb is no more than 5% in diameter not .5"

This is the best tire calculator:
http://www.miata.net/garage/tirecalc.html