One thing that I did notice about 205/45x17" tyres is that most of them are Extra Load, (XL) rated - or worse,
Runflats, which indicates stiffer sidewalls - although Bridgestone's T005 in this size are an exception and quite
a good, quiet tyre IME - when fitted on a relative's mid-size 1600 MPV at least.
A drop down to 16's or even 15" certainly opens-up a much wider choice, with tyre size 195/50x16" maybe an
ideal overall compromise to retain looks, being only 2.6% larger rolling radius compared to 215/40x17" - again
a quick search shows that Dunlop, Bridgestone and Yokohama offer suitable non-XL tyres in this size.
SEAT themselves supplied Ibizas with modest 185/60x15" through to 215/45X16", although neither of these
extremes seem to be an ideal size to be the best possible overall choice.
205/50x16" is another option that would fill the arches even better, but at 4.4% larger in diameter - and although
perfectly safe - is getting near to the sensible '5%'
guide limit that most tyre experts advise.
Whatever is chosen, a modest 1.4
Ibiza is not going to need any tyre speed rating higher than 'V' or a width
above 205 for a car of this weight for road use, unless looks/image are the be-all/end-all.
As always, you have to inform the insurance about changes, but most underwriters are not concerned about size
changes within such sensible limits, providing both rims and tyres are fit for purpose. Another member on here
sometime in June 2019 changed his wheels to aftermarket 15" alloys on his Mk.3 Leon, complete with deep
sidewall all-season tyres with no insurance impact and swears it improved how the car drives no-end.