Did you check by moving the transmission counterweight by hand into gear?
I've had many clutch failures but never had it just slip like that. Not saying it wont. Maybe its a Competition clutch and its wallowed out on the shaft and stuck open.
Is there any noise in the clutch area?
I guess if you've been launching much at that power level you could have stripped the output shaft against the differential I think.
2007 Exige S with almost every track mod...