You will not get a warning light as there are no sensors directly on the calipers, pads or discs. The only sensors you have is brake fluid level.
I havent really noticed a difference with regards to the handbrake but it depends what you are looking to do with the handbrake (normal use or drifiting) i only use it as a handbrake.
Well the ABS is still too intrustive on the Supra in general IMO & there isnt a huge amount of difference from stock, but then how early ABS cuts in is down to how good your tyres are really. All ABS does is it senses a wheel lock up and that is caused by your tyres loosing grip under hard braking, upgrading the brakes isnt going to make you stop any faster as thats down to your tyres, bigger brakes will allow you to do repeated stops (race track or twisty country roads) without brake fade, but to stop faster you need to also upgrade your tyres,
I have have the 8pot front and 4 pot rear on 356mm discs all the way around.
Read my review on it...http://www.mkivsupra.net/vbb/showthread.php?206375-K-Sport-Brake-Kits&p=2934804&viewfull=1#post2934804