Err, apologies, I was a bit brusque in my initial reply. Sooo...
I've got two '01s with ABS, and on them thus far I've run:
275/35-18s and 335/30-18s (stock size)
5.0x5.25-16s and 26x11.5-16s (drag race slicks & skinnies combo)
11.5x?-18 and 14.0x?-16s (goodyear roadrace slicks)
and the ABS has worked flawlessly in all cases.
Now with the Forgeline 19s, I'll be running 295/35-19 and 345/30-19 Pilot Sports
The tire circumference for those is 85.23" and 85.29", compared with stock circumferences of 80.36" and 81.41", for a difference of 6.06% and 4.77% respectively. So you are correct, the ratios aren't exact; there is a 1% difference in the ratios with the 19" Forgelines vs the 18" Stockers. However, with the forgelines the front and rear circumference are almost exactly the same, which oddly enough is exactly the situation encountered when running the drag slicks & skinnies. Thus, I do not anticipate any ABS problems when running the Forgeline 19s with the above tire sizes. If I encounter any, this board will be the first to hear about it