I've lived here (villages and for a bit in town) on and off my whole life, for my sins.
I'm not sure how building a Waitrose on the Junction for the M40 will help bring house prices up? Not a criticism or anything, I just don't see how it could? John Lewis didn't have that impact... If anything, I would say that the increase in traffic on Junction 4 will knock them down!
Knowing WDC, they would use any subsequent traffic problems as a way to justify the new entry/exit slip on to the M40 as that's what they really want. That will send the prices near to the slip road plummeting, along with reducing the villages around it into even worse commuter rat runs than they already are.
If WDC put as much thought into developing the town centre properly, as they do into the out of town shopping areas (which isn't a lot, tbh) then the High Street would not be in the sorry state it is now.
I don't think High Wycombe is becoming 'gentrified'. The 'worse' areas of HW are still just as bad as they ever were and the infrastructure cannot support the number of houses. It has always been a commuter town, that's nothing new and is the reason that the villages are as expensive as they are.
In fact, I would say that the opposite is true of High Wycombe. I find it quite sad to see. I feel like the town centre is slowly collapsing in on itself and that the town is a mess of ill thought through, rash planning department decisions. Sorry! Though the shops in Eden are better than they used to be in the Octagon/Chilterns 
That said, if someone wants to try and convince me otherwise then I willing to listen. I live in hope that this place will improve! It's probably hard to be objective when you've been somewhere for a long time...