I've tried two 30-06 Improveds, with disappointing results. One was a 24" barrel, the other was a 26". I didn't have any of the new wonder powders though. I also had a 22" 280AI ( reamed out Mod 700 Mtn rifle) it was finicky, only liked two bullets, the Nosler 150BT and the Speer 160HC. I got 2950 with the Nosler and 2800 with the 160. Only about 50fps better than standard. It was a worthwhile experience, and I and a buddy killed lots of game with it. I wouldn't want one with less than 26" today. But that's just me. One great benefit I see of any Improved round is you can load to "optimum velocity" of the standard round but with less pressure. 50-100 fps over the standard would begin stretching primer pockets in the old 35 Whelen. With that long throated Remington I gained powder space by (a) seating the bullets way out and (b) Ackleyized. Sure, you can run them pretty hot, but, as said, at a risk. I did the same thing with a Mod 700 Classic (24"bbl) in .257 Roberts. It was my "first" experience with the Ackley. I had it set up right with a crush fit chamber for standard rounds. I seated the Hornady 120HP way out and "even used" Ackleys loads from his book! Wow! I never chronographed it, but it had to be humming, ha. It was a death ray on crows and deer in East Texas! Lots of fun. Needed? nah...I went on and played with the 25-06 ( very, very good 25 caliber round) and the "real sweetheart", the 257 Weatherby...."now that's a 25 caliber"! ha. My only gripe with the Weatherby is the soft Norma made cases. Federal wasn't much better back then. I didn't even try to make them from .264 WinMag WW cases, but that certainly is an option.
