I find the biggest drawback of Feudalism that it comes later. But if you've got it available and you're still in Despotism it is worth analysing the situation. I recommend looking at the economy tab of CivAssist for that. Comparing Feudalism with Monarchy is easy, as they both have the same contribution to science and the same MP allowance (3 units per town). So you can simply look at how many units each government would support, and pick the government that'll save you the most money. War weariness should hardly be a factor if you're stronger than your opponants. Comparing with Republic is more difficult, as that's quite a different animal. Regarding the cats to trebs upgrade: I've never noticed much difference between the two types of units, so in itself this upgrade may not be value for money, but looking at the whole upgrade line cat > treb > cannon > artillery it does come as interesting. You don't want to still be carrying around catapults in the Industrial Age! You do want to upgrade your artillery, even if the first step is not that fantastic - it doesn't cost more if you're upgrading in more steps, by the way.