Ive got a Y Section after the Cats which then runs a single pipe to the Mid and Rear Boxes on a Performance Stainless Cat Back System on one of my 3.2 Plods, despite Peeps telling me over the years that you need to retain the Twin Pipes to at least the Mid Section or through out, it does actually run very well, good performance and not too loud, more of a deeper rumble.
Trick is with a Y Piece is to make sure the single Pipe Cross Area Volume can accommodate at least the same as a the Twin Pipes its is replacing.
As for decating, I wouldn't bother, otherwise your need to faff around with miss reading O2 Sensors, you do need some back pressure before the Mid Section anyway, and then theres the MOT Man to worry about once a year.
I did try a Decat on a LPG Car, plan being I could present the Car on LPG with out the Cats as it would require a looser MOT Test over Petrol, but it didn't really run right and sounded bloody horrible in my option, so put it back as with the Cats as it was.