Need to get 2woody on this case. On the face of it, you haven't changed the piston ratios much front to rear (but remember that opposing pistons do not add extra area). Not sure how that would affect the balance calculations - hopefully not much.
What he's saying about ABS systems isn't really valid. ABS or not, you want the brakes balanced, because if (say) the ABS cuts in early on one axle due to an imbalance it will affect the effectiveness and balance of the brakes dramatically. Also, in cornering, before lockup, a rear bias can still cause it to swap ends without warning. EBD in the ABS system can be used to proportion braking instead of a bias valve. The jury's out on whether the Omega has it or not. I think 2woody reckoned it might have, given the piston sizes at the rear of the later DBW cars. ABS ECU calibration could be completely wrecked by this change, too, since the sensitivity of the calipers to fluid pressure changes will be very different, and that's not something we can tweak.
The other thing to consider is that you haven't changed the master cylinder to match your increased piston area. You might well find the brakes to be over-assisted or, worse, that there is so much travel in them that you don't have enough reserve travel in the pedal.