Watch out for American stuff. I believe that California has quite strict emissions requirements, so "more cats" to meet that requirement in the yank market is not unusual.
Regarding:
It is also interesting Some of the Brands like Invidia, AVO, Cobb, etc sell 2" up pipes. It makes sense if more gas can enter the turbo quicker it could spool up faster.
Note my comments from the other thread:
Keep your wits about you when considering any aftermarket larger-diameter up-pipes.
The whole point of a turbo system is for the exhaust gasses to be funnelled into a very tight nozzle within the turbine housing where they can impart maximum velocity in the turbine wheel. The tightening begins earlier - noting that the cross sectional area of the up-pipe is not double the cross-sectional area of each exhaust bank's merged output.
If you swap out a factory up-pipe for a name-brand aftermarket large diameter up-pipe (but otherwise keep a stock-ish sized turbo etc.) you'll just have this large diameter, lower-velocity exhaust hitting the flange of the turbine housing and having a sharp 90 degree edge as the point of reduction to go into the turbine housing.
A larger diameter pipe will flow the same fluid/gas at a lower velocity. See for example your garden hose with no fitting on the end versus a restricting nozzle that lets you squirt water 20 metres at much higher velocity. Which hose configuration could spin a waterwheel faster do you reckon? Same principles apply for turbos. Look at the snail shell shape of a turbine housing - designed to focus the gas through the smallest orifice in the whole exhaust system (disregarding the wastegate) at maximum velocity to impart energy on the turbine wheel.
Larger diameter up-pipes might have their place for Subi owners running massive turbos, but for a street car running a street-style turbo I recommend much more research before getting out the credit card on "bigger is better" principles.
Also, aftermarket up-pipes seem to leave it to the user to figure out heat shielding, with most solutions being sub-optimal compared to the factory dual-layer stamped steel heat shield in my opinion.