cobra imolas, or preferably Suzukas (they fit my arse better!)
the suzukas are around 6kg each (i have full carbon at 4.9kg, but then i'm a show off!)
but stupidly i have mild steel subframes! i must get round to making some ali ones
std seats are around 25kg each, so 50kg total, you are replacing these with seats,sidemounts, subframes + harnesses which in reality i would estimate at around 12-14kg per seat (complete with all the bits)
so you're basically removing 22-26kg in total
that my conservative, realistic answer, its still cool saying i have seats that are only 4.9kg!!