You have to be joking right?

17's completely louse the cars handling, no ifs, whys, buts or maybes, they just do.

I know of more than the odd person who has insisted on having 17's (or Escort Cossie rims) then when they've had them on the car and driven the wheels have been sold on almost immediately

Your Si would benefit massively from a reduction in rim size and raise the ride height to gain some travel in it. This may be why you noticed no difference as it appears you've just slammed the car as low as you can get it without thinking how it will affect the drive