In my humble opinion...
If you have hubs that are languishing, they would make great experiments! You have already put the work into creating the basic hub, now go back and experiment with different ways to improve them, and then watch the results.
As to spending time improving the popular ones... well that doesn't really make sense. It they are popular, they really don't need tweaking, do they?
I wouldn't remove a hub unless it got really bad reviews and so on. What if it just isn't the right season for it? What if the topic suddenly becomes hot, and you've deleted it? So, for me, (and I haven't got a lot out there yet) I would rather just let them sit for a few months and see if something pops, or tweak them and measure the results. Once I've put the effort into creating the hub, I don't want to trash it.
Good luck. =D