I have the pipe you refer to, in combination with a standard centre pipe (with the only silencer of my system) and Alfaholics front pipes (which didn't really make any audible difference to OE front pipes).
I think those websites are covering their ar5es with their 'racing only' comment. It's in no way too loud, mine has passed an MOT without any problems, has been on for a year and when I was pulled over by the cops on an unrelated matter no comment was made about the exhaust; I would lay money no cop or authority would have an issue with it. I love the sound it makes, I'd never go back to OE and to this day is still the best thing I've done to the car as with every drive the car sounds fantastic.
I was in St Albans today, not a million miles from you misterchase...
Ex '99 6-speed 3.0 V6 GTV, now Z4MC