I just wanted to tell you all that after a whole lot of research and comparing images of the A3 and Leon exhaust systems i took a chance and ordered a Fox cat-back system for the A3 8L 1,8T Quattro and it plugged in just perfectly! Mounting points between chassis and exhaust components lined up perfectly with eachother. The only thing that bugs me a little is that the tailpipes hangs a bit lower than the ones on the OEM exhaust system.
However, the rear muffler was a "duplex" one (tailpipes on both left and right side) so I had to cut off the tow hook and a part of the chassis where the tow hook was welded on to and the bumper needed some rework for the right hand side tailpipes.
So for the conclusion, if anyone else wants a noisier exhaust system on their 4wd Leon 1M, the A3 8L Quattro systems seems to be one plug-and-play option, unless you order a duplex one.
Just for a quick
review, the Fox cat-back with mid-silencer delete pipe is rather quite during cold starts and during normal driving conditions. It starts to make noise at around 2000 rpm and kinda goes a little back and forth between loud and quite up to 3500 rpm and over that it's kinda hard to know if it's louder than the OEM exhaust system. It makes small exhaust pops sometimes as you change gear at around 3000 rpm. It feels (or sounds) well balanced. It did however cost me 630€ so it was a bit pricy, but in my opinion worth it.