Spend the time/money and make or buy a decent merge collector, those formed collectors are disgusting, they make a decent difference to power right across the rev range. Also with a merge collector you can run a smaller collector diameter with a megaphone and reversion cone. This has the upside of increasing velocity (venturi effect) which has a good effect on mid range torque with out losing topend. Really it comes down to how much time and money you want to spend. But even just having a decent merge collector will make a decent difference

There was an example (i think it was edlebrock) where the only thing they changed was the collector on a set of headers to a merged collector from a formed collector, no other changes to the header in terms of diameter or length (i can't remember exactly what it was some big yank v8 with about 530whp) and the thing picked up about 20-30 whp, a substantial amount anyway. So that would probably equate to about 3-5whp on an A12 I guess for not a lot more money.