You only need a few mm of clearance to the strut tube. Most of the tire deflection occurs at the bottom contact patch. 8-10mm is plenty.
Have you considered a 10mm spacer rather than the 5mm? This would still place the 9.0" wheel not as far out as the stock ET23 9.5" wheel. The 9.0" wheel with the 5mm spacer will effectively become a ET30. This would put the larger 9.0" wheel 7mm closer to the strut tube and place it 5mm more to the outside fender lip. This is a good fitment and keeps the centerline of the wheel at approximately the same place on the front. Using a 10mm spacer will give the wheel an effective offset of ET25. This would place the wheel 2mm closer to the strut and 10mm closer to the fender lip. In comparison, the stock 9.5" ET23 wheel on the front would be 6mm closer to the strut, but 18mm closer to the fender lip, or a little over 3/4". At stock ride height, this may be OK. It would be interesting to see if the stock 9.5" rear wheel/tire rubs under agressive manuvers.
The reason I ask about the 265's on a 9.0" wheel is that a 255 R compound will be wider than most street tires, and the 255 is a better match for a 9.0" wheel. That said, I think the 265 on the 9.0 is definately doable and not a mistake, just another alternative.
The only thing that has not been discussed is if the rolling circumference being equal rather than a variance of over 1/2". Stock front is 25.7" and stock rear is 26.3", or a difference of .6" with the rear rolling slower and the front rolling faster. The computer was probably calibrated for this difference in rolling circumference. What will happen when the split is taken away??