Hmmm - I used 140gr. Noslers in both my 7x57's at 2,940/2.950fps. I would think the .280 Improved should do about 3,150fps with a top-end load with a 24" bl.- maybe better depending on the rifle. Heck of a good plains game ctg. I've never liked the .270 - maybe Jack was the reason - but I've always liked the 7mm's - except for the Rem Mag. Had a couple of those - NIX.

Googling .280 should get some good data for load development. I'd merely start with the slower powders and max loads for the standard .280 and go from there. Max loads for the smaller case are the place to start, not below that when using slow powders.