Gents, I have a leak in my downpipe due to a crack/corrosion at the y joint between the two pipes. The easiest option would be to weld it up but I was thinking that the single pipe 2 1/4 " that goes past the gearbox is a bit small for a 300 plus hp motor. I might see about replacing it with 3" or whatever can fit or a true dual system. I would prefer dual as expect it would sound better, but would be harder to make given the confines. Any opinions? I have high flow centre cats no downpipe cats and standard silencers.

 

To follow up. After looking under my car and considering the already limited ground clearance and space past the gearbox (GM4L80E), we decided on a single 3" section.
The downpipe joint was cut back to give almost full diameter at the flange going into a collecting cone on the new section.
The rear of the new section was reworked to give a smooth, full diameter split into the two 2 1/4" pipes and cats. Not easy with all the angles but my good old Dad is an experienced fabricator and did a nice job of it.
There was not much room for a heat shield so I used some insulation where it goes past the gearbox, which may also help reduce noise and warm up the cats.
The pictures show the old y sections compressing the pipes to half their diameter, so it should help flow. I haven't timed performance yet, but it is a bit louder than standard despite keeping the same standard silencers, only a bigger diameter pipe, but not annoying. It sounds better than the leaking original, with a deeper rumble at idle and no blowing. On the motorway you can hear a slight drone but only like a 'normal' car, and mine has more revs on due to the 4.01 gears. With the window open it is a bit louder than stock, especially under load. Sounds ok though.

 

I have a completely dual system with no downpipe catalysts, modified under-floor catalysts to provide twin entry and standard under-floor and rear silencers. The exhaust note is lower frequency due to halving the gas pulse frequency through each silencer, but not actually louder. I like it.

 

I remember you posts on the matter, and largely the reason why I would have preferred dual pipes - I even bought the tubing to do it. Unfortunately I think the Auto is a lot wider at the sump than the Getrag manual so space limited. I also seem to be struggling for ground clearance on my car, despite replacing the springs and fitting the 4.5 mm packers on each end plus new original dampers - but that is another story.