Re: The "I could have dunked" moment...
I think everyone has days every now and again where they feel a lot springier than normal. I know I've definitely had those days myself and I always try to re-trace my steps and figure out exactly what led to this result. At one point, I traced it back to hot dogs because that was what I ate the first day I dunked on someone (honestly, thought eating meat prior to playing was what did the trick). Another theory has been riding a bike. I think there might be something to that, where my legs warm up before I even hit the court, thus introducing my max bounce right away.
If you're consistently 4-5 inches above the rim, maybe it's time to try reaching even higher with the ball in your hand. I definitely know that in games, the adrenaline combined with being forced to push myself as hard as possible can sometimes push me higher above the rim than when I'm just warming up and shooting around. You should plan on that and maybe develop that finger roll if you haven't yet. It is possible to dunk when initially taking off and planning layup, but it's likely not going to be ferocious and the true likelihood is that it will end up looking like a finger-roll-rim-grab.