Of course you can mobilize your best tank divisions armed Leopard 2A6/7/7+ and even new "Revolution MBT" but you can't win a few of regular regiments of imperial guard, because one regiment is equipped like all your divisions, not in quality, of course, but in quantity
As for lascannons - these aren't the only energy weapons, there are also plasma cannons and ancient volkite weapons, the latter two would have no problem breaking through the armor of any modern tank. But even a lascan. can disable, for example, this very Leopard 2. the Lasgun does not need to calculate ballistics, it fires hurricane bursts that can destroy the observation devices and the running gear of the leopard without hindrance.
Heres a Spookstons video talking about things wrong with the Leman to slightly back up my argument. https://www.youtube.com/watch?v=e8n-cw08YYA
Ofcourse my knowledge of the Warhammer universe is almost non existant and thus i could be wrong and i surely am not ready to read 20 books just to proof that the Leman Russ is a bad fighting vehicle compared to Late WW2 or modern era vehicles.
Judging by this i could argue that the lasscannons couldn't penetrate the almost half meter thick armour of a Leopard 2 for example.
Also seemingly the Lascannon cannot penetrate or has a hard time penetrating the armour of TAU mechs which seemingly use a more sophisticated armouring scheme compared that to the flat 150mm of Plasteel on the Leman russ itself.
I tried to look up information about the Lascannons. It is said that they are antiarmour weapons and that they are used to destroy tanks. But remember that when they say tanks and armour they don't mean modern MBT:s but Warhammer tanks and armour. Which atleast on Leman Russ:es case doesn't seem that impressive.
kerponaut dont forget about other modifications of Leman Russ that have energy weapons that do not need the space of the combat compartment to reload and can destroy any tank up to modern models
also you can see very good draw by russian author: https://sun9-49.userapi.com/c850136/v850136785/13d785/iZtv2uojzic.jpg