No it does not sound as a diesel...
I was convinced (as most youtube listeners) that it was a great thing to do ....how wrong I was.
The sound and resonance in real life is completely different to the wonderful sound on youtubes.
The standard resonance is on an MKVI around 1700 rpm...hardly noticeable and most of the time in a non working environment (eg idle)....changing the exhaust in anyway will put the dominant resonance in a different area...e.g with a smaller Simons Muffler in lets say 2700 rpm....
I think ( make that I know) that area means trouble.....eg when you drive 120 km per hour in 6th the dominant resonance will not only be much louder (unmuffled or smaller muffled) but also constant.....

It might be a totally different ballgame if you change more parameters ( cartuning) but to me the best overall result is standard exhaust....