Luke, you can swap the front brakes for those from a GS450h - or any other Lexus models that feature those same ADVICS 4-piston calipers.
They're a direct bolt-on replacement with just a small piece of the dust shield needing to be trimmed to accommodate them on an IS.
Pair them with a set of GS450h discs & pads and you have a significant upgrade. Furthermore, the GS calipers are a monobloc design which makes them maintenance-free with none of the anxiety that they'll eventually start sticking and seizing up.
Maybe, but I agree with Linas in that they feel kind of weak, even in perfect working order. The front disc diameter is only the same size as a 1st-gen IS200, which weighs over 200kg less.
Anyway, I'm curious to know what you guys mean when you say braking/stopping "power" doesn't get increased with bigger brakes?
If you mean stopping distance then yes I would agree, but for me, braking power is most definitely increased. The brakes are more powerful and require less physical effort (on the brake pedal) to achieve the same level of retardation.