The main limitation in a 1U chassis is the size of the heatsink + fan. They're typically 80x80mm width/length, and the height is very small. You get 45mm for the whole case so you have to work with a slim (15mm) 80x80 fan and a tiny heatsink that's 10mm or so. So they have to work at 5000rpm and put out 60dB of noise but even so they're typically only rated for 80-100W.
I other words I really don't think it's worth bothering for a 250W draw. If you can get a 2U space yeah, that gives you much better cooling options.