I'm not sure why you'd change from the Fullsac to the V&H or what you would hope to gain by doing so. You certainly aren't going to gain any performance by simply switching the headpipe, in fact I think most people on this forum think that the Fullsac is a better product. I can't say that with any certainty, I do have the Fullsac DX on my bike but I did go the extra mile and have Steve tune the bike based on the onboard equipment and it's wonderful. Could he have got more or less out of the V&H pipe? I seriously doubt it, there may be a slight change in the curve somewhere but I'd bet there's no way it would be significantly better because I've seen probably a hundred stage one dyno sheets from correctly tuned bikes and they are all within a few ticks of each other.
If you want a real difference, stick with the equipment you have and let a real pro have at it with a dyno, for what you'd spend on that V&H pipe to gain virtually nothing, you can ditch the stage one canned map and get it dialed int throughout the powerband.
Just my 2 cents. I think V&H makes some nice stuff, but I'm not seeing any value in doing what you propose to do. To answer the original question, I think if you just switched the pipes and left it as is, it would run virtually identical to how it does now.