I ripped out all the N249 gubbins today and reversed the dv to see what effect it will have. These were my results:
boost spiked at 20 psi then held at 18.5 psi
boost peaked at 22 but then it gradually dropped off to 15 psi by 5500rpm!
I'm guessing that my N249 was faulty because I used to get a fluttering noise that has now gone but maybe the reversed dv is leaking boost.