[The HACK]

I run the rear wheels up front with moderate negative camber on 245/40/18s, no spacers. Maybe the fender and the fasteners have already been worn away by the 275/35/18 on 9.5" wheels (the front offset of 43-18mm=25mm).

Here's what I see as the problem. At 30mm offset it's just in far enough not to rub the fenders. At 28-3 (effectively 25mm) and at 9.5" vs the 9" on my setup, you're effectively pushing the wheel 11mm closer towards the fender. With the amount of additional caster on the Z4 M front suspension due to the offset FCAB, when you turn the wheel full lock the outer radius increases and the tire comes in contact with the edge of the bumper and fender liner.

You have 2 options. Ditch the spacer. If you need 3mm to clear the spring perch, remove the helper springs. That pushes the KW club sport base perch up by about 1-1.5", which should alleviate the need to use a spacer to clear (ask me how I know). Then run centered FCAB to decrease the caster angle so as the kingpin swings from one extreme to another, the edge of the wheel won't swing out and rub and damage the wheel well and bumper. You lose a little bit of the sharp steering that's inherent on this chassis, but you gain more clearance.

Or run a shorter front tire. 265/35/18 is about 0.4" shorter than 245/40/18. The front end is spacious enough to accommodate up to 285 wide tires with minimum rubbing (again, ask me how I know). A 265 width tire is narrow enough to still be a good fit on 9.5" wide wheels, and being .4" shorter would alleviate that 11mm closer to the fender you're dealing with with this particular wheel/tire combination even though it's WIDER.

Of course, neither combination is guaranteed to fully alleviate unless you try it. I just know that some here have had good experience (myself included) regarding front rub with either. I am 99.95% certain the combination of ditching the 3mm spacer (and removing the helper springs to clear wheels/tires inside) and going to a non-eccentric bushing will solve it (heck there's a pretty good chance just removing the spacer AND helper springs will 'help'). I'm 85% sure going to 265/35/18 will solve it despite the math about the tires being wider but shorter.

You won't know until you try though. But MOST here are correct, your front offset for the size of wheel and tire is way WAY too low. You need about et 38 for that combination to work flawlessly inside the front wheel enclosure...But if you can't and won't change the wheels, those two options I outlined above is what you should try.