1999 996 C2 4% Rule and ABS

Vehicle Information: Model: 996; Year: 1999; Body Type: Coupe; Total Mileage: 70K kms;

I'm geting ready to purchase a nice set of used 18" 997 Carerra III rims/tires to replace the stock 17" wheels on my 996 C2. My reseach on this site tells me that the rubber I'm getting ready to purchase exceeds the 4% rule. The front tires are within spec (235/40/18) but the back tires exceed (265/40/18). I think my calculations are correct and I kind of like the thought of a higher profile on the back for comfort but I don't understand the ABS problem well enough and AWD does not affect me. What kind of problems can I have with the back rubber 1% higher than recommended? Your comments will be much appreciated as always.Thanks