oooh, it's very possible you blew up one of the resistors on your card - making your card identify itself asa quadro FX which it isn't; now, this opens different pathways in the driver, that the chip can't follow because it isn't exactly the right chip, neither is it the same RAM - may explain the slowdown.
Congrats, you just turned your Geforce into a workstation card 😛
Worst thing is, yes, you did null the warranty when you overclocked it over manufacturer's settings. Get yourself a new card if you want to, but keep that one: it looks like it is working. You can also try to use Rivatuner to disable the QuadroFX optimizations - this may solve your slowdown problem.