Not sure what you mean by 17" space saver. The space saver spare is the same for 15" and 17 . My car is on 15" but my Auris was 17 the spare was the same. The boot poly is 3 piece, it does make the boot shallower, but there are cut outs for Jack & brace. There may be good reason why the wheel well is so shallow but it beats my logic. The pump and kit went with the Auris and I sold the space saver on EBay.
On 15 " the car is quiet,economical and a great drive. The Auris Ts was good to drive on 17" but struggled to get 50mpg. I had a loaner Auris hatch on 17" that was very noisy, skittish to drive and still struggled to get 50mpg. My understanding is on 15" it will not attract congestion charges anywhere and be in a different tax band when or if introduced. On fast cross country mixed motorway and A roads I get 65mpg. That's 15 mpg better than the Auris Ts on 17". The only criticism of 15" is they do not have rim saver bands and have an odd plastic cover to the Alloy wheels. The test drive cars were all on 17", one of the three I tried was quiet,an Excel, the B and B+ road noise was noticeable. I put this down to tyre choice and luck but it may have been insulation.
For me it was a no brainer save £400, get a better quieter ride, and better fuel consumption, but we have been here before. Other than looks I can not think of any reason for low profile tyres. Let me know if you need pics