I think lexus had a long think about this and decided knowing that the rear brakes do less then 30% of the braking it wasn't worth it to add the extra rotating mass and unsprung weight to the drive wheels. To compensate they went nice and big in the front to give you enough stopping power. Increasing caliper size and piston volume will give you less pressure and might even hurt your braking performance. A lot of people have issues with front brakes warping but no one really seems to have issues with rear brakes. I'd just throw tt calipers up front. You could try for bigger but there doesn't seem to be a lot of people who have done it so I'm unsure the stock master could provide enough flow to make the larger size worth it.