I have been working out what is different between the SP and FS, in addition to the LAW38 conversion pack and there are a couple of points if you want to build a TRUE FS.
1) rear wishbone inner hinge pin mounts LA271 are specific to FS (not sure the difference so if anybody can tell me great! I can not get these parts in the UK at the moment)
2) rear hubs are the UM519 parts from the RB5 worlds rear end, standardisation at last...
3) the rear universal swingshafts are the 62.5mm ones from the original RB5 (UM510).
Apart from that there are a couple of minor changes (mainly including option parts like the steel pivot balls in the kit) but nothing that will effect the on track performance.
Hope this info is useful to somebody and that somebody can tell me the difference between the LA271 and normal hinge pin mounts, if I find out first I'll let you all know!