Originally Posted by Madjack
Legally, they have to prove the modification led to the failure of the part, but from what I have heard...that is not hard to do.
Correct, though it really depends on the dealer. However, because you are dealing with a BMW, you can purchase Dinan parts which should do nothing to the warranty.
Also, keep in mind that the modifications you listed would only void the warranty for those particular parts. In other words, just because you modified the springs does not mean they can not honor the warranty to your engine, brakes, etc.
With the parts you mentioned on replacing, you would have no problem with BMW in replacing them. If the springs fail for some unforeseen reason, I am sure that the manufacturer's warranty will cover everything anyway. The only time you should really worry about your warranty is if you do anything directly with the engine. Modifying the ECU is a classic example. However, if you changed the exhaust (common modification), it would take a lot of miracles for the dealer to prove that the exhaust modification caused engine failure (or what have you).
Bottom line: you will be fine with those mods.