There is no easy way out of this one. Also the flanges on most header exhausts are very wimpy at best. The slip-on collectors are not going to work. No leaks allowed. If your turbo'ed, the answer is no because you will always have back pressure and never scavenge to any degree that will make a difference.
Bigger diameter slows down the speed of the exhaust and also cools the flow entering into the turbo.
You want the exhaust to be as short as possible because you need the heat. The idea is that the exhaust is still expanding when it comes out and you want it to do that inside the turbo, this helps spool things up. Some turbo books say that it's not the flow that makes the turbine spin, but the kinetic energy in the leading edge of the exhaust pulse, or the shock wave that makes it work. If so, that explains why they want the smaller exhaust pipes to keep the speed of the pulse up. Bigger tubes make volume tanks and will average or slow down that pulse.