I have heard the WP8, WP7 and WP6. The WP 8 are clearly better than the WP7 and WP6. More coherent, organic sound, and better, more refined highs with less bite. It's a slippery slope to make a WP7 buying decision based on hearing WP8, IMO.
My gut tells me anyone considering any iteration of Watt/Puppy loudspeakers will not be satisfied buying Von Schweikert loudspeakers; not because Von Schweikert wouldn't be satisfactory in every regard, but because the buyer will always be aware of the nagging "what if".
Are you going to be able to compare WP7 with VR4 SR loudspeakers? Unlikely.
So, what to do?
Pick one, and buy used. I'd suggest starting with WP7. If you have any objection to their sound, then sell them and buy used VR4 Sr.