The answer to your question is like the subject of recoil - the answer to which is "it varies from person to person."
Generally speaking, most shooters probably won't be that affected by the muzzle blast from a .280 with a 20" barrel. I personally have shot a .270 with a 20" barrel and, while I prefer a 24" barrel, and the muzzle blast would be less with a 22" barrel, the muzzle blast from the 20" barrel was tolerable. Given that a .270 is very close a .280, I would find similar results.
Again, it all varies from person to person.
Zachary