1lb per plant is possible, sure, it's just that you have so many limiting factors.
And yes draw that cold outside air into the pump that will help a lot as long as it doesn't go too far the other way and make them too cold, you'll have to keep an eye on it.
How big of an air pump? How much money do you have to spend? Regenerative blowers are what people used to get max yield before high water flow dd style but they cost over 1k. DD's pump would be good, if you can afford it, for just your flowering girls only.
I will just be running one of these for my 4 bins to start:
I couldn't afford the hi-blow yet.
They don't have the power to push crazy long runs under pond deep water like the hi-blow, but in a small shallow setup they will still throw a crapload of air. If you used that just for your flower plants you should be able to froth the water just like it's on a high boil. It should be really rolling hard and if it's not, then you aren't in that range of ideal yet.
It's all up to you man, you get out only what you're willing to put in. Much of what I've said to you, you should take with a grain of salt though. I was coming into this as if you wanted the most productive growth but it sounds like you aren't that concerned and just want to do the best you can with what you can. I get that... I'm in the same boat to some degree.