The larger power LED lamps are available from any specialist lighting shop here (in NL) but they never seem to appear in a supermarket or normal high street place.

The larger LED lamps (yes, I don't mean CFL) I have used in light-fittings still seem to have a short but noticeable turn-on time, perhaps a tenth or a fifth of a second. I have no idea if that is consistent or not, but it may be an unwanted variable for short exposure times. Apart from the location of the illumination with respect to the condenser(s) of a black-and-white enlarger, the other question-mark would be about the spectrum of the colour temperature of these lamps and whether there are any peaks or troughs compared to incandescent lamps. The only thing to do is try them and see I suppose!

I'm half wondering about using one of the industrial, high-brightness LED's as a pseudo spot-source but I'm uncertain about the effectiveness of a pre-existing condenser in this case. Another thing that has to be tried out probably. Perhaps there will be a metal-work project swapping for my De Vere Varicon lamp-house this winter . . .