If the clutch is slipping, the engine will rev with good power all the way to redline -- it sounds powerful. Your description of no power and "limp" makes it sound otherwise.
So help me with my original question: does the engine spin up and sound healthy, but the car doesn't move; or does the engine bog down and not run with any power? These are VERY different things -- which is it?
Fluid under the master is never a good thing, and you should investigate that regardless of anything else. But loss of fluid would leave the clutch fully engaged, rather than disengaged.
There's little chance of clutch fluid boiling, since it's never close to a source of large amounts of heat. Also, clutches are less prone to effects of embedded air, since the pressures are low and flow is large (as opposed to brakes, where pressure is very high but flow is low).
Bottom line, hydraulic problems can make it impossible to disengage the clutch, but generally won't make it stay disengaged (or slip). If the clutch is slipping, it'll be a mechanical problem instead.
BUT, there's an exception to every rule. If your clutch pedal is adjusted too high, it will close off the port between the reservoir and the master cylinder. That permanently seals the hydraulic circuit, so that when the fluid heats up (not enough to boil), it expands and starts to release the clutch. It's easy to check, and worth doing. Just reach under the car and try to move the rod coming out of the clutch slave cylinder with your thumb. It'll take some stiff pressure, but you should be able to make it slowly move. If it's just stuck in place, then look to the pedal adjustment.