Fast Intentions exhaust is here! (continued)

V1H

Hi Tony, thanks for your helpful explanations.
I still have some questions.
I notice a distinctive design difference between your 2.5" and 3" TDX.
The 3" seems to use 3" TP in place of the downpipes, with the catalysers (100cell) now moved downstream into the lead pipes. On the 2.5" the cats still sit in the downpipes (factory positions), the lead pipes are thus straight (or resonated).

I wonder why the cats moved into the lead pipes. Is it just more temperature-friendly? And since the 3" uses testpipes only, does the O2 bung extension on the TPs still prevent CEL?

I'd be all up for going 3", not 2.5" if the only difference is better Turbo spool.
Any sound differences considering both non-resonated?

Most importantly I still wonder whether the 100cell cats pass M.O.T. here in the U.K.
Problem is I don't know how modern cars are emission tested (I've had an OBD1 Prelude before) and my Z is still too new for M.O.T
Do they actually sniff the emissions on the tailpipe (then the 100cells would surely fail?) or does the ECU merely give a pass/fail in which case one could trick the ECU into satisfaction?