Is it better to clutch for a panic stop? Maybe, but not for that reason. The engine has some inertia, but I doubt the car can stop faster than the engine. The car takes a very long time to stop, the engine natuarally drops in revs pretty quickly. I always hit the clutch at some point during a panic stop because I don't want to stall the car. Also, the extra braking power from the engine (I think it probably does provide resistance, even on fast stops) will bias the brakes even more to the front. They are reasonably well-biased from the factory, but err on the side of too much front. If you have a lowered, more stiffly sprung, or more rear-biased car, more front braking power isn't needed. Besides, once you peg the brakes, you have all the power you need to stop. Those discs can lock up very sticky tires without too much of a problem. But that's all academic. Really:
It's a non-issue. Leaving the clutch engaged until you are about to fully stop (or are in a spin) is the best idea, just to keep vacuum pressure up. Downshifting through the gears isn't a bad idea, partly to keep the revs (and vacuum, and engine braking power) up, and also to be able to floor it if some idiot comes barreling in behind you. It won't take long for a good driver to get it into gear and go, but any time at all can be too much in an accident situation.
