Current bike has XT brakes. Came with SLX IT rotors
No it didn’t. It’s the aluminium sandwich construction for better heat dissipation which makes a rotor “Ice-tech”, and that only comes on XT, XTR and Saint level rotors.
SLX rotors are RT-67 for centrelock, and RT76 for 6-bolt, and are solid steel. XT rotors are RT81 for centrelock and RT86 for 6-bolt and are Ice-tech.
Similarly, its the finned pads which make a brake “Ice-tech”, there’s nothing different about the calliper.
Thankfully, the rotors and finned pads are interchangeable across all of Shimano’s latest lineup of brakes, so you can mix and match to your hearts content.
Personally, I started with non-icetech rotors & pads, thinking it was all just a marketing gimmick by Shimano. But my brakes were overheating at the end of long descents, so I switched both bikes (one SLX, one XT) to all Ice-tech and haven’t had a problem since.
To answer the OP’s question – would Ice-tech rotors make a difference? Probably not in ‘normal’ riding, but given the small difference in cost it’s good insurance for that one time your brakes overheat on a long downhill!
As for sizes, I’m no lightweight and run 180 front and 160 rear on both bikes. The current Shimano brakes have stupidly strong stopping power, and I couldn’t see myself wanting more, unless I was heading off to the Alps – and then it’d be for heat dissipation again not stopping power.