Hi Rolf. I've loaded .22-250's since the late 1960's, on and off for many years. I've never had a problem with case stretching, but will say, that the straightened case will likely stretch less. IIRC, I trimmed cases every 6 or 7 firings, to .010" shorten than max. The last .22-250 I had, I sold a few years back and was a Remington that did not shoot well enough to suit me. I had loaded that brass only a few times, so didn't have to trim it. A 25" or 26" bl. will work just fine for this round and deliver top speeds, as in 3,900fps with 50gr. bullets, maybe 4,000fps. In the Improved case, these speeds would likely be 100fps to 150fps higher.
I have liked all of my Ackley IMP. chamberings over the years, so if you like them too, then you will be happy with the .22-250IMP as well.
I don't understand why re-proofing is necessary for merely shortening a barrel, but I guess if that "back" has to be scratched, so be it.