I guess I'm in a good position to comment on this! I have a Janspeed cat-back exhaust on my Lupo GTI, not the huge tail pipe version either, exhaust note is perfect, not too loud that it gives you a headache, but loud enough you know it's there. The bore is the same as factory so it doesn't damage any of the back pressure you need, the fit was perfect and was worth every penny of the £350 I spent buying it.
I also have a 2019 Up GTI in Tornado red, to say it has no character is doing the car a huge discredit. Sure it's no Lupo GTI, but the car has bagfuls of charisma and I'm loving every drive out in her. Handling is fun, performance is more than adequate, and I'm hoping to remap her soon for around 140ish bhp, plus with an induction kit from Forge Motorsport, I'm hoping for 145-150bhp, which is more than enough for a car of it's size. I highly recommend it!