End of the day an item is only worth what people are willing to pay for it, so if its worth 300quid it will get bid up to 300quid, regardless of starting price.
Just need to ensure it gets maximum views, so get the categories and all the specific details correct, ensure the title and description is good, and matches popular search terms, and make sure it finishes at a sensible time.
That ones finishing at 8pm on sunday which is pretty much peak time/day for ebay visits. Title and description could be better though. I'd have put "1.8T" and "1.8 T" in the description for instance, as if you searched for "A4 1.8T Cat" it wouldnt find your auction. It also doesnt state which model of A4 it fits, could be B5 or B6...
i wouldnt expect you wouldnt lose any torque, as the turbo spooling earlier will more than make up for it.
It will also make the car pull a little harder for a little longer, as the cat becomes more restrictive when your high up the revs, especially on a chipped motor.
Our car had a gutted cat on it when we baught it, and we refitted a real one for the MOT, dawn doesnt drive it particularly hard, but i think there was a slight difference, with the car feeling a little flat compared with before.
Its about the going rate, its just a **** of a place to get to on the car, and swapping it out every year for an MOT would do my head in.
What i'd probably do instead, is get a turbo back system and one of those cheap sport cats from ebay, then fit that under the car using two of those audi exhaust sleeve clamp things. That way its easy to swap out for a bit of streight pipe, and easy to swap the cat back in come MOT time.
Them rip of merchants they charged me £90 off for mine when they were that price on ebay, i tried to get the difference refunded and they refused. And would not accept the refund if you want it dont order it from their site only use ebay or there rip you off.