My most recent B10 racer(recently sold)went 820kg. with me in it,fullcage,and everything gutted that was legal,A15,toyota gearbox,1200 ute diff etc.So if your looking at MX5 i rekon you'd be looking at about 900kg.
That said unless your going thru the firewall and going Sports Sedan regs.Wait you are going s/sedan anyway cause your not keeping the original manufacturers block type in the car.Back to it now..unless you go the firewall and get weight of the front the thing won't handle,the biggest problem with all the B10's is the chassis flexing from the fire wall forward and the pissey little front towers where your top wishbone bolts.The movement in these areas is massive and adding weight is a recipy for disaster.These areas can be modified to a degree but won't achieve much by way of stiffness but will add more weight.If you go full blown s/sedan regs you can fabricate towers and poke some adj.struts in place of the transverse leaf,but now you have a trailer only car which you will not get registered for road use(it is possible i suppose,but i think a new house would be cheaper)i do regular crack tests on the standard uprights/stubs with no problems to date on either car.
Hope all my personel experiances help
Cheers
Bert