It will depend on a lot of factors like your stride mechanics. I'm a lot slower than most of the people on this board--lady who runs ~1:33 half, training for sub 3:20 (maybe 3:17-3:18) marathon, and I find them to really work for me. I've used them from a 10 mile race through a marathon. I got my first BQ in them. I would say they are definitely faster than trainers (as you would expect from a flat), but how much faster they are compared to other flats is going to depend on your biomechanics. Prior to vaporflies, I raced in Nike Lunaracers which are nice and light but really lack energy return. I have a pair of original generation Zoom Flys (not the new Flyknit ones) and while I like then for workouts, especially tempos, and they have some of the forward propulsion from the plate, it really isn't the same as the lightness and pop you get from the 4%s. The 4%s also are light without having the beat up feeling you get from lighter, more responsive shoes. For what it's worth, I'm 110 lbs, so I seem to be able to get away with less shoe than some of the guys I know, but I still find it to be a nice plus.
I have no idea how much I get time I get in the vaporflies versus other flats, because my fitness is much higher since purchasing them. If nothing else, they are just really fun to run in, at least for me when I'm going marathon pace or faster and have a really fun bouncy feel. I think they are worth $250 for the enjoyableness of the ride alone and the combo of lightness and cushioning (and lack of a beat up feel after a race), since after all this is just a hobby. I think of it is a sunk cost--I'm going to need to wear shoes to race, and if not vaporflies, I'm going to buy another pair of racing flats. A lot of nicer racing flats are $100-180 or so anyway. So let's say I get seven or so races out of a pair of flats, that works out to an extra 10-20 dollars per race that I'm spending using vaporflies over another pair of shoes. IDK, I can live with that.
As for whether I would be able to achieve my current PRs in other shoes, I have no clue, nor do I particularly care. Some of my PRs lately have been on hilly hard courses, or in rain/heat, or tune up races training through without a taper, so I'm confident there are a myriad of factors besides shoes that affect performance on a given day, but it's a nice psychological boost to put on some shoes that are fun to run in and make me feel fast.