I think a lot depends upon exactly WHAT point of measure we accept as the “immediate gain” - for sure, what *I* measure immediately out of the tube is NOT what I will measure a week later, with no further pumping.
BUT, after steady and longer term sessions, there WILL likely be significant, more permanent gains that WILL remain. I pump at higher vacuum, and for far longer lengths of time than most - and have made about a 1 inch permanent gain in length, and at least the same in girth - but nothing really close to what I have right out of the tube - and right out of the tube is what I frequently use with my wife - and THAT is currently 7.5 length x 7 girth. A few years back, when asked if she would enjoy sex better if I was a bit larger, my wife replied “sometimes” - and now, it takes me a full 5 minutes to get fully inserted - with LOTS of lubricant and steady strong-but-slow thrusts before she stretches enough for normal action - this morning she stated I was getting “too BIG”! She was referring to girth - I still only faintly detect anything that might be “bottoming out” - so am now putting together a longer tube - I’ve outgrown the 7.3x2.5 one I have been using up to now - I’m seriously shooting for 9 inches BPEL - but doubt pumping alone will get me there.
I’m quite fortunate that my wife is fully aware and supportive of my efforts to cope with ED and use of the pump - and with my gains, has been increasingly interested and cooperative in frequent sex - something she never was when I was smaller - I’m fully convinced that bigger IS better - and makes for FAR greater satisfaction and interest for BOTH sexual partners.
I only wish I had been aware of PE decades ago!