Firstly, we need pictures. BBS CH are gorgeous wheels, I don't think I'd ever change them but I was sneaking a peak at the BBS site the other week to see what they could offer
And an extra 0.5inch shouldn't really make much difference (so the ex told me
) as with the same offset, it'll push the wheel OUT.
The problem is the offset of the wheel, as its bringing the wheel further into the arch and catching on the plastic. I don't know what the offset of a standard 156 is, but I'm guessing its different on a GTA due to the wider track...
You have two options, firstly to fit spacers. I wouldn't be happy with 5x100's on wobble bolts and spacers....so I wouldn't recommend this.
Secondly, as you say, you can fit narrower tyres. This will give you exposed rims, so you need to be careful on curbs! Curbing BBS rims is sacrilidge
*Personally* I'd be asking the garage to sort it out - they've recommended you some wheels which aren't fit for purpose, I'd ask for some in the correct size/offset.
But again - pictures!!