I guess what I'm looking for is someone to tell me that if I do this my ABS will still work correctly...
Sorry to be so needy....
To answer you question - yes the ABS still works.
my measurement tells me that the ABS will take a fraction longer to unlock the wheels than stock setup. I can remember the average additional stopping distance with ABS activation as i did this 2/3 years ago - it was not a lot but certainly measurable and repeatable at speed up to 80km/hr.
- the theory is that the ABS is "calibrated" to release X amount of fluid per "lockup detection" based on stock caliper setup - larger piston area (ie 4 pot) require larger release so an additional time was required to release more fluid
Of course the above is a hypothesis as exactly how Bosch ABS system work is a closely guarded secret. Note Bosch engineer tells our german friend Castle Tek that the mod is a "problem"
I am sorry to be the black sheep, but i am only telling you my measurement and my experience. Most people wont notice the difference
If you are keen to do the mod but are worried then i suggest you do it in steps.
1) clean and lightly sand the disc and change all fluids and put a lightly sanded EBC HH pads to ensure you are getting the best efficiency out of the stock;
2) change to larger brake M/C - you will want to do this if you change to 4 pots anyway. If you this point you are happy with the change go no further - do some brake distance measurement deliberately jamming the brake to let ABS do all the work
3) change to 4 pots compare the braking distance relying purely on ABS, factor in the additional brake feel and make a decision - with so many people doing this mod, i doubt you will lose much if you change back to (2) and sell the items
good luck and let us know your decision and measurements