It depends whose going on offensive and whose on defense.
If CL has to invade a Imperial castle....then the Imperial's will own.
If the Imperials have to march up to "The Fort". CL snipers and guerrillas will decimate the Imperials before they arrive.
Assuming those snipers don't get hit by fireballs and die horrible horrible deaths from being burned alive, or the Imperial Legion doesn't simply overwhelm them with their vastly superior numbers, or send in an invisible mage to make one of the snipers go crazy and attack people at random. Assuming the Imperials don't continually send weak summons until the legion runs out of ammo.
Or strong summons which are immune to non-magical weapons.
Let alone the fact the Imperial Legion would vastly outnumber the as they basically are ancient Rome in size.
Here is the hugest problem with the argument you Caesar supporters have.
Caesar's Legion is able to hold its own against the NCR,
Relying on low-tech, numbers, physical fitness and discipline a lot of there guys don't even use guns but simple swords and other low-powered melee weapons. the NCR has a lot more technology and guns yet they can't break the stalemate against the legion. Saying the imperial legion can't over come the gun disadvantage when Caesar's legion has done it for the past decade is complete and utter bull. By your logic the NCR should have crushed Caesar for good at the first battle of hoover dam.
Caesar's one solitary advantage is a technological edge, but that didn't ensure NCR victory did it?
In the end Caesar wouldn't want to fight the Imperial Legion he'd want to join them as their basically everything he admires about ancient rome[plus they have magic]