If you contact JeffyB and James Rowe, I believe they have done this on their GTV's.
The Cybox Decat Pipe is not that expensive and again the emissions bit will vary from car to car, depending upon how its running etc. It's probably a case of fixing the pipe, running it up and having the CO2 measured to ensure it won't fail at MOT time.
If this doesn't work for you, then I am sure you'll get most of your money back on the pipe in the AO Classifieds.
Also, as a side item in my reply to your PM about the Throttle Body I have for sale, I forgot to mention that if you do buy it you will need to have the TB Sync'd with your cars ECU and a Specialist will be able to do this with AlfaDiag - unless you have the software and cables to do it yourself. Cost can vary from Free to £25.00.
If you have the "P" codes read, normally the TB failure relates to a code description of "Motoriser Throttle Control Failure" (or something along those lines) - Fuel Injection Light will be on and it will feel very lumpy on tick over and won't rev beyond 1,000rpms. You will also need to check the MAF, as sometimes these issues can be more than one root cause.
The item is already pre-packaged and I will be able to ship within x1 working day from receipt of payment. If you want RM Special Delivery by 1pm, then add on £5.00 to cover the upgrade, otherwise it will be on a x3 day economy service. Cheers - Paul
1999 GTV - PH2 3.0 V6 - Vela Blue - 'Blue Style Momo' - Aero Pack
GTA Clutch, PH3 Alloys, Ragazzon Quad DTM, Koni SR-T, Powerflex Bushes