Actually I think I have a better idea where everyone wins. I think what you should do is to model the Marian army as the one of the late Republic during the era of the civil wars while the Imperials should be modeled on the army of the 1st century AD which was slightly different. That we can can have both a more average legion and a more super legion.
The army of the time of Marius would have had an influx of raw recruits so it would make sense that it would have many people but not stated as highly. Basically like more slightly better Hastati (call it ~1.5K-1.6K). The army of the 1st century Principate which had experienced many civil wars and reorganized into basically a super border patrol should have the the legion as a battle hardened nearly elite unit (~1.8K-2K) with many cheaper support units (~1.2K-1.4K) to help out.
There's probably no harm in keeping the legions 200 strong while reducing support units to smaller sizes (spear auxillaries to 160 and archers to 80-120?) since atleast wikipedia quotes documents that put legionary formations as large units with auxillaries in much much smaller units.
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