Ive been researching this as I need new pads and rotors on my 03 SR5. Seems the best solution is the Gen 5 calipers and rotors. 338mm rotors which are thicker than the Gen 4 338mm rotors and the calipers have 3mm larger pistons than the gen 4s.
Unfortunatly I found out after ordering the Gen 4 Stop Tech Z36 kit and lifetime warranty calipers from Auto Zone. Also make sure you replace the old rubber brake lines with upgraded stainless steel. The lines make a substantial difference with brake pedal feel/modulation.