"Stages" don't mean a great deal as one company might sell a stage 1 for one power range and another would have this as their stage 2.
There are quite a few clutches around, if you are planning some power upgrades then make sure the clutch can handle this. Heavy clutches can be a pain in traffic so although it sounds good to get a 900bhp rated clutch you will end up with a real pig of a drive.
You might also be getting transmission losses, its not an auto is it?
To test the clutch hold it at a steady 30 in second gear, then press the accelerator hard. The speed should go up with the revs, if these rates do not match then you are getting clutch slip.
While you are changing the clutch you might want to get a lighter flywheel as well, this will help it rev more freely.
On these cars the ECUs components can fail (resistors I think), its worth investigating this and replacing the PCB components ASAP as after they fail you will need a new ECU and this can be very expensive.