I'm going to chuck my own hat in the ring here, I've been using K&N replacement air filters since 2007 without any ill effects whatsoever. Ok, this is only my own experience, but only two weeks ago, I took the one I have in my Saab 9-3 out, washed it and re-oiled it using the correct K&N recharge kit and there's no issues.
It's like anything, there's so much out there for and against. I didn't ever use one in our previous TA or the POP, because both cars were in warranty and indeed, I won't put one in our i10 because that has a five year warranty and it's on a service plan.
As for using one on the TA, well I would feel somewhat cautious because of the engine technology. On our previous POP, for the standard 1.2 engine, I'd have used one, no problem. On something like the standard common rail Fiat diesel engine fitted in my Saab, not really an issue, nor was it an issue when I had one fitted in my Chrysler a few years back. Indeed, I actually still have that K&N cone filter for my old Chrysler, still in excellent condition in its original box, sat in the garage, must sell!
Personally, if you want to use one, go ahead as long as you understand what if any benefits using one might give. As I've highlighted before, I wouldn't use one in a car still in warranty, but that is me.