This can get very long winded, I wish AO had made a sticky covering this. I've tried to answer this many times but all the calculations etc just get lost in the old threads.
It's the same for GTVs & Spiders.
If we just use 6.5J 16 & 7.5J 17 as the 2 std GTV/Spider wheel sizes it will help keep things clearer.
Too many people just post 'I've got 20mm, or 15mm, spacers fitted' without stating whether they're on a TS or V6 & without giving exact wheel ET, J & tyre size.
It's the rears that give the clearance problems & the TS runs a 24mm narrower rear track, 12mm each side.
A 6.5J is 1”/25.1mm narrower than a 7.5J, which therefore sits approx 12.5mm further out & in.
A 6.5J 16 can be spaced 20mm without problems, even on a V6, but although a TS can still space a 7.5J 17 by 20mm you can't space a V6 15mm without clearance problems.
To add to the confusion, over the years it's also possible that rear hubs bought from breakers etc have been wrongly supplied & fitted.
You tried spacing your 7J 17 by 15mm which rubbed & your wheel's ET of -35 sits 4.5mm further in than a std -30.5 ET wheel, plus being a 7J not 7.5 it is another 6.25mm further in so it should've cleared. Either you haven't got any bump stops fitted &/or the 225 tyre on a 7J is shaped differently, I think 215/45 is recommended for 7J 17.
As a guide to help calculate clearance for a V6 I found that after deducting the spacer thickness, & also half of any increase in wheel width over a 7.5 17, from the wheel's ET that I have clearance at -21.75 but not at -17.75. So somewhere in between the two is the maximum you can go. That's using 225 tyres, any wider it's back to 'guesstermating'.
Using this method yours came out at -26.25, also strange it's only the osr on both cars?
I wouldn't worry about the front track, whilst waiting for my narrower 12mm rear spacers I ran 8J 18s ET -40 unspaced rear, 16mm spacers on front........it drove absolutely fine.
Genuine GTA wheels should clear Brembos without spacers, afterall they were/are designed for the GTA, also aren't genuine ones a slightly larger ET? Perhaps you've got the 8C copies which are very similar?
I said it could get long winded! You could've just phoned me.