May i ask why your wanting to keep backpressure ? on N/A yes it can help alot however on a turbo car the exhaust is litterally just to get the gasses out ASAP, in the turbo honda scene in the states alot of people will just run a bit of pipe to the ground or even out of the bonnet ! ie Remember the less back pressure the better on a turbo application And stepping down from 3-2.5 after downpipe isnt a massive issue as gas speeds will be high down there anyway, its getting the gas out of the turbo thats important, although a 3" exhaust would help massivly at them power levels.