The Russians have always been good with armor, most likely they will be fitted with ERA (explosive reactive armor) which is a bunch of stick-on explosive boxes that defeat the kinds of armor-piercing warheads called HEAT rounds. Although the Russians claim to have ERA variants that work against our 120mm APFSDS shells (basically, a depleted uranium dart an inch and a half in diameter moving 5,000 fps), we've never come up against anything that will stop the main anti-tank round from an M1A2, and no M1A2 has been knocked out by a frontal hit by another tank. Ever.
Ultimately, the extra 5mm on the Russian tanks doesn't count for much, the US wins tank battles by seeing the enemy first, hitting them with APFSDS from up to 1,000m beyond their maximum range, and being able to survive any frontal hits the enemy can dish out. Our fire-control systems are better and our crews are much, much better trained.
The problem with the gun-missile system you describe is that if you're in a T-90, my 120mm main gun will fire three times and probably hit you with the first two before your single missile flies to my position. 1200m/s beats a 450m/s flight time any day. It may be helpful against helicopters, but again, the helicopter will be replying in kind. We don't have to worry about that because we own the air before we fight, and the M2 Bradleys that accompany the M1A2s have 25mm chain guns will do simply unspeakable things to an airframe.
I'm impressed by your armor knowledge; thanks very much for your post, very interesting!
The T-90 has a Russian APFSDS shell. The real difference is optics. We can identifiy them from beyond 4km, they cannot. We still have first shot, first kill, but combat is often chaotic and identificationmay take time, especially since blue on blue incidents are seriously frowned upon. At 2KM or less, I would preffer that our enemies have a t-72, T-55 or worse.