Somethings wrong and its likely to be something simple (it nearly always is in these cases).
Firstly you can eliminate the offset - as your spacers take care of that.
Secondly you can eliminate the lowering - as mentioned this doesnt affect the clearance to the damper.
As far as I can tell it has to be either:
1. The profile of the tyre
2. The design of the Koni spring
Common sense would dictate its more likely to be the spring as this is the non-standard part - perhaps if you are really unlucky it could be a combination of spring/wheel/tyre/offset.
Whatever the answer is I fear for you thats it not going to be cheap. The easiest way is probably going to be a wider spacer - will switching to a 20 or 25mm not solve it?
Failing that a different profile tyre.
Failing that change out the dampers for OE.
Im running 225/45/17 (lowered 30mm) on KYB gas-adjust dampers (supposed to be OE spec) and theres plenty of clearance to the damper, although it does get VERY tight in the wheel arch.
-1998 916 Spider 2.0TS Lusso-
H&S Corse stage 1.
CATCAMs stage 1 cams with Savali custom ECU remap.
305mm discs with 166 4-pot callipers.
Ragazzon exhaust mid section.
Biscione Racing 2x70mm rear section.
17" Zender Siena wheels with 3mm rear spacers.
etc etc etc!