I bought a .280 Handi last week, and finally got around to shooting it today. My first group, and this was out of a clean barrel, went into less than 1" at 100 yards. The load was factory 140gr or 150gr Remington Core-Lokts (not sure, I lost the box). The velocity was around 2900fps, but I need to check my paper to get the exact velocity. The only other factory load I tried was the Federal Premium High Energy 140gr Trophy Bonded Bear Claws (that's a lot of capital letters!). Its rated at 3150 on the box, and three shots averaged closer to 3130fps out of my 26" tube. They also went into an inch at 100 yards, so this gun is definitely promising!
I tried a few loads with 4350 & 4831 behind a Speer 145gr spire point, and the gun shot better with 4350. I got several 1.25-1.5" groups, and even a 1" group, but velocity wasn't what I was looking for. However, I didn't really push the loads to maximum, and the fastest velocity was right at 2750. 4831 was much faster, and I got almost 3000fps with a below max load. However, it looked a little a hot and accuracy wasn't great. I need to play with both powders and my OAL, and I think I can get a 3000+ load under 1.25" with no trouble.
As far as the .280 cartridge, it is my favorite. I have hunted with a Remington 700 Mountain Rifle in .280 for over ten years, and it has taken quite a few deer. I don't think there is a better deer gun available, and the .280 can cleanly take game out to 300 yards with no trouble.