i JUST HAD A 280iMP done. They feed like butter in my 700. Yes you need different dies.
Load with 5% under max load for parent. Nosler has some info. 22-250IMP is one of the more common. Nosler and Seirra will help you out
Expect a 5-8% gain (more like 5%). Increase the powder accordingly. Very accurate while forming. Make sure your reamer is cut to allow .002-.004 crush fit on the shoulder.

Pros you get a round not many people have.
fireforming is really cool
You do not trim as much
Brass lasts alot longer
You can grab factory loads and go to the range or
PD town. Make brass there.
A fast twist barrel you can sling heavys 55gr plus

Cons Dies cost alot more
harder on barrels
Loads for fireforming will be different seating depth that the everyday loads. POI will change from F/F to everyday loads.

Would I do another, HELL YEAH
I have a 17Ah, 280Imp in the cabinet. A 17-222 improved 32deg and a 20-222 IMP 40deg shoulders at the smith now. I have a 25-06 that is getting sold for a short action to do a 257IMP or something with sharper shoulders and no body taper. Unless I get my hands on a 6.5-284 reamer.I love them.