From previous experience I had 15mm spacers on the rear (fitted by previous owner) but they ruined the handling. Once removed I found the car met my expectations.
I had removed the spacers when I lowered the car (30mm). With the rear track 30mm wider the outer edge of the tyres was outside the wheel arch and on bumps the tyres would have hit the wheel arch.
I was keen to get my replacement wheels to sit close to the position of the original teledials. With 10mm spacers I have them 4mm further out, whereas 15mm spacers would make them 9mm out and 18mm wider rear track.
I understand the reservation of the thickness of 10mm spacers, but I think it is unfounded. Once the spacers and fitted and the bolts torqued up the only force acting on the spacer is compression between the two faces of the mating plate. The outer hub carries no force and is used purely for locating purposes. And even if the outer hub of the spacer were to somehow separate from the hub once fitted, the wheel-mount would still be secure.
10mm spacers on GTA wheels with ET35 will give an equivalent ET25 which is 5.5mm wider than a std 17" Tele(ET30.5). Total track increase of 11mm.
GTAs with 15mm spacers....ET20.....10.5mm wider.........track +21mm.
Teles with 15mm spacers....ET15.5.....15mm wider.........track +30mm.
Forget the lowering as that confuses the issue, basically a 30mm drop is just the same as the first 30mm of suspension/wheel movement of a standard car. The tyres would've rubbed but more likely inside the wheel arch rather than hit the lower edge.
If you want your GTAs to sit where std Teles sit then fit either 4 or 5mm spacers but these can't be hubcentric & therefore you only have a small 'locating' lip. This lip is for more than locating, it helps takes the strain off the wheel bolts. That's why reducing spigots are fitted to aftermarket wheels with larger centre bores. Also that's why 15mm hubcentrics are stronger than 10mm due to the amount of metal where the lip is.
As for the handling being adversely affected....I'm running 8J
18s ET40 with 16mm fr & 12mm rr spacers giving wider tracks of +25mm fr, +17mm rr & it feels fine.........actually excellent.