firstly is the cost of stainless, i know they last longer but there is a pretty big jump from mild to stainless.
stainless is more prone to cracking when there is rapid heat change, eg. driving when raining, hot pipes with cold water being splashed on them
exhaust gasses flow quicker when hot, thicker the pipe the hotter it will stay for longer so the gasses will flow quicker with ease. so im fairly sure that the mild exhaust pipe is thinker (not by much).
plus i know people may say they look better but in my opinion who cares its under the car there going to get dirty and what not as soon as you drive the car
stainless is stronger than mild but does not like to flex (why is doesn't like to have rapid heat changes) so in my opinion having a bit of flex is good for example if you happen to hit something with them, stainless has more of a chance to crack while mild would have a better chance of dinting then cracking.
these are my opinions and your free to have your own but this is just the way i see it.
as for the size, one of the ways how you can make extractors is to make each one of the primaries have smaller capacity (ml) the the capacity of the cylinder (ml). the theory with this is that when the gasses leave the primary (1) at the collector it will create a vacuum in the other primaries(2&3) thus sucking the gasses out of the other primaries causing there to be nothing to slow the gasses down.
these are what i have come up with and last time i talked about making extractors i had the usual reactions so mine will be this way they may work they may not but ill give it a shot

the ones i will be getting will be,
1 5/8" primaries at 295mm long
witch will give each primary a capacity of 394ml so that will create the vacuum
also the hole on the exhaust gaskets are 1 5/8" (could be 1 or 2 mm bigger or smaller) from memory.
then 2 200cell 2" cats
2" secondaries at what ever length is left to fill to connect to stock piping
anything will be better than stock anyway in my opinion.
on the full twin exhaust, there may be a benefit in it but i am not sure if there is or not i have read of people trying to change the exhaust size to stuff like 2.5" the splits into twin 2" so its like the stock but bigger and they have lost power, so it could make a difference but might take a bit of r&d. i have seen someone on here do it in a exhaust thread i think and he seemed happy. i personally wouldn't do it for the cost because it would be fairly expensive i would assume.
i have been thinking of changing the whole lot when i get the extractors or maybe even a bit after, but i would be changing mine to a single 2.5" system with a decent sized resonator and decent muffler just to give it a crisp note and not to loud whilst keeping it fairly straight thru, and if i don't like the look of a single muffler i would put a dummy one on the other side.
hope this helps
