"The T-90S gun can also fire the 9M119 Refleks (NATO designation AT-11 Sniper) anti-tank guided missile system"
This is an interesting deviation from NATO tanks which cannot fire a ATGM. The US used to have the Sheridan and a version of the M60 with a gun that fired a missile but that weapon system was a big disappointment. THe US and Germany worked on a tank that was suppose to fire a missile but that program, as almost all multinational tank programs was cancelled. The 125 MM gun is larger than the standard 120 MM NATO tank gun. I am not sure if the T90 had special armor like NATO tanks which involve use of ceramics and other materials.
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.