I'm kind of surprised that it would be that drastic a difference. I could see an issue having a log on one side and running a header on the other, but who would do that ...right? LOL I agree with the guys. I'd wait too. I mean, why not do it all at once. If it's at the shop and on the hoist...
I didn't buy the HP manifolds because everything I could find out about them suggested that I would only gain 5 or 6 horses over the logs, assuming duals. It stands to reason that if the logs were running through a single exhaust, then you installed duals when you installed the HPs, you would see higher gains. I also came away with the impression that the HPs were more prone to cracking than the logs. The other reason I didn't bother was because when I installed duals with turbo mufflers on my 390, which was just running basic manifolds, I was very impressed and satisfied with the gains. So much so, that I decided against the headers I'd been considering.