Rotor size has nothing to do with brake bias. Larger rotors don't get as hot and the same amount of pressure from the pads has more effect on a larger caliper. Brake bias has to with the effect that the work that the master cylinder is doing in form of hydraulic pressure, has on the caliper pistons in the front and the rear.
Typically when you go with a larger caliper (bore diameter or number of bores), you need a larger master cylinder so that more hydraulic fluid is displaced.
The Brembos will work fine with the OEM master cylinder. And noone has tried the 4th gen mod of the STi brake booster and master cylinder on a 5th gen yet. From the best I have been able to determine, the early STi master cylinders has a 1-1/16" bore diameter, but the 08+ seem to have a 15/16", which is the same as the 5th Gen LGT. So dunno.