I tend to make all the rest of my siege full out Barrage, since if I'm attacking a city with siege I want to do maximum possible collateral damage.
You may want to rethink that. The amount of extra damage you get from the Barrage promotion is a small fraction of the amount of collateral damage produced by an unpromoted catapult.
The problem is integer math. Collateral damage uses collateral strength, rather than combat strength, in its calculations. Collateral strength is base strength plus collateral bonuses.
The promotions of the barrage description lie (at least the english versions do). What really happens is that the collateral "damage" number listed in the description is a percentage bonus added to your collateral strength.
So a Barrage I catapult goes from strength 5 to ( 5 * 120% = ) strength 6. A Barrage I treb goes to 4.8 collateral strength... which of course gets rounded right back down to 4 (no effect!)
A Barrage II treb is strength 6 (150% * 4), so that's all right. A Barrage II catapult gets screwed (7 instead of 7.5).
Once the collateral strength of the attacker and defender are known, the game then figures out the damage done - and this process includes another integer rounding (aka a jump point).
So in an example like "catapults vs longbows", if you set up the experiment in world builder, you'll find that an unpromoted catapult inflicts 9 collateral damage to each longbow, and that a Barrage I catapult inflicts 10 damage.
Improving the survival odds of the catapult are a much better investment.