Its the tyres rubbing on the body/guard liner, not some diff issue....

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...

Refreshing this thread, my 2006 gt b spec is doing exactly the same thing after crowe sport replaced a coil lead. Any advice on whats going on?

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. 

If I look at a 22B for reference, they run 8.5" ET48 wheels, so surely 7" ET48 would be fine with 225-wide tyres. I guess the issue must've just been the difference in overall rolling diameter