It will be the width. GC8's don't like 225 wide tyres without rolling gaurds. Even though rolling diameter is the same due to offset the wheel has moved 7mm further out and then the tyre has gained another 10mm in width to the outside.
Go to a 215 tyre and it will likely be fine.
If you have lowered suspension this will make gaurd rubbing even worse.
I am running 215/45R17 tyres on ET53 wheels and no rubbing at all.
The 22B also has HUGE gaurds compared to a stock GC8, making room for the extra width.
I'm fairly sure a 2006 Legacy is coil-on-plug so won't have coil leads?
Could be heaps of things. Boost control solenoid on the way out, ECU pulling boost due to detecting a fault.... does it idle nicely? Could be a miss...
If all 4 are the same diameter there is no diff issue. Only the speedo will read out by the increase or decrease in diameter.
wider wheels when turned travel inwards towards the frame more, so can rub. Or the rear bumper mounts where it connects to the wheel arch can rub too.