> "I've already got a pickguard cut for a TELE-NECK-SIZE middle pickup, no screw holes, courtesy of Warmoth."

Not sure why you didn't opt for the standard Strat-size middle cut-out, but so be it. You've limited your middle pup options, and will have to accept slight pole-piece misalignment.

> "I used to have a 280S set in a Strat & found the 2 & 4 positions were excessively muddy compared to the single-pup sound, even w/500k pots."

Such a problem is almost certainly caused by inappropriate height adjustments. There are few pups as mud-free as Bill's, and none I know of are more mud-free, or as organic.

> "I notice the 280 Tele neck is brighter & clearer than the 280 Strat neck"

Under the covers, so to speak, they're essentially the same pickup!

Aide toi et le ciel taidera (Aid thyself and heaven will aid thee)