Google brings up a lot of S65 for sale but the ones I find a B40s
http://m3forum.net/m3forum/showthread.php?t=373552 for example
S65 202 kg (445 lb)
N54 195 kg Both according to wiki
so negligible difference weight-wise.
Best of luck if you go through with it, you will likely also need new ecu units specific to the engine if I am not mistaken.