Performance wise, a Swift does not benefit all that much from "improving" as other cases do such as the 22-250, but longer brass life from less stretching is alway a plus.
I'm certainly not doubting you, I personally wouldn't know either way as I've never had one, but I do know a Swift improved picks up about 15% in case capacity, which is a lot, where the 22-250 only has about a 7% gain in case capacity. Just based off of case capacity I don't see how a Swift Improved wouldn't have a significant improvement over its parent case unless it were with light bullets and or powders that weren't slow enough to take advantage of the improved case.
I sometimes wonder if people think this because they are using some of the old P.O. Ackley data where the slowest powder available back then was 4831 or the like.
I think with modern day, much slower, powders that are available and the heavy for caliber bullets we now have, a 220 Swift Improved should run nearly exactly the same as the 22-6mm and maybe even a little faster.