I can only offer from my knowledge of instructing on VGS in the late 70s but I was impressed that we managed to solo more young pilots each year than the RAF regular training system. Whilst the RAF did not accept many of our youngsters that was their loss because all of ours were well motivated and gave up much of their spare time and cash to fly with us. I have no doubt that many of them would have served happily to 55 as long as they were flying - and that represents a much better training return than the RAF gets from most of its' pilots.
I believe money spent on VGS is money well spent and I would rather recruit from there than any university.