It sounds like a good idea to me. This would give the ultra low BB a good raise, would steepen the head angle (and so enable the option of running longer fork without messing withe the HA too much, and while were at it raise the BB even some more
), and , as a bonus, squeeze some more tavel from the bike.
I cant really see how the VPP sag is so specific. On the swinger the recommended sag is 25-35%.This is not so specific range. Running a longer shock with 35% sag would be fine I guess. Since I have a fox shock like that running around at home, I think maybe I'll give this a try
edited : on second thought, If I remmember correctly from the 4.5" famous threads, the frame itself cant handle more the 5.5" travel - the swingarm reaches the seat tube. Guess I'll try the longer shock anyway.