I believe there are a couple members here running that round so they should be along shortly. I was going to go improved but the gunsmith had a regular 7-300 norma reamer so I just went that route. Not sure when my barrel will be here though. I'm thinking BJay has the improved version.
What type of information are you seeking? I don't own a 7-300 NM Improved, but I have shot one a bit (I do own both 300 and 338 NM Improved rifles). Sierra 197s at 3250 with RL33 and a 30" barrel. Pretty brisk recoil in a hunting weight rifle. In general I am a huge fan of the Norma mags. If you are wanting an extreme 7mm, the 7-300 NM would definitely be my choice. Barrel life will obviously be extremely short, but anyone building a 7-300 knows what they are getting into. I would go with a minimum of a 28" barrel to really bring it to life. 30" would be better.
I have a 7mm-300 Norma imp 40 degree built on a Stiller tac 338 I had it built with 2 barrels . It shoots amazing will last a long time as a pure hunting rifle . It has a fluted 30" barrel 12lbs with March 3x24 scope . I do not think I would go 40 degree again but at the time the gunsmith had the reamer and dies . As with all overbore calibers shoot it cool and it will last. It sure is a flat shooting wind cheating cartridge. Shooting steel with it gives you confidence to take ethical shots on game at longer distances it truly has a ballistic advantage running 195 bergers at good speeds. I ran the numbers though JBM before building it and out to 1000 it smoked nearly everything while still being comfortable to shoot. Enjoy!!!
I would only fireform 50 or 100 depending on how much you will shoot that rifle. No point in doing a bunch if you are only going to shoot 25 or 30 when you take it to the range. 100 pieces should last the life of the barrel and fireform loads can still be a great load just a little slower. Hydroforming gets it close but you will still have rounded shoulders that need fireformed from the research I've done on it. I considered it for another ackley cartridge that I have.
Did load work up on mine yesterday and worked up to 96.5 gr of rl33 with 195 berger and it yielded 3384 FPS, I probably could have gone a little higher but on high temp Louisiana days this could get into pressure. Humidity was around 60% and temp was 70.