The K&N doesn't need replacing at service time just cleaning and re-oiling every 50k miles more expensive at £51 initially but as it doesn't need regular replacement cheaper in the long run. If they fitted it as standard less profit for them lol
The fitting of a free-er flowing airfilter will provide limited performance benefits without various other modifications to the intake and car and even then the gains would be minimal. You'll certainly see no more than a 1-2bhp increase. There may be a marginal benefit to mpg but it'll be so subjective you wouldn't notice the difference.
As JohnC mentions the major benefit of fitting something like a K&N is that they do not need replacing and with the appropriate kits can be cleaned at service intervals. K&N already do replacement panel filters for most PSA vehicles so chances are if there isn't one already there will be.
Fitting of induction kits however is to improve performance and I think we need to remember the Cactus isn't designed for taking corners flat, go pedal buried into the bulkhead at with full lock dialled in. If you want more performance a remap would be a better option but will have warranty implications. Both mods also have insurance implications.
Totally agree you wouldn't see much difference, and as Rich says the cactus isn't a sporty number.
I used to have an Alfa 156 2.5 v6 cloverleaf with a BMC CDA forced cold air kit coupled with a 2.5" stainless steel exhaust, now that was a different beast lol. And those modifications definitely helped, but I did spend a lot of time in petrol stations if I opened her up
Totally agree with both comments, why buy this sort of car then think about turning it into a monster. Our 110 is plenty for this type of car, but in no way shape or form is it a 'performance car' or ever will be. My 1380 custom minivan has more oooomph!!!!, stoppings a little tricky though on 7" Cooper 'S' brakes lol
Unless you have a way of closing off the induction kit to engine heat fitting an aftermarket one will just cause it to get heat soaked and lower performance. Your best bet is either a replacement panel filter or a dedicated enclosed induction kit and even then the performance difference will be minimal on a Cactus!