One large plugin vs multiple smaller plugins
I don't believe there's any significant impact in having multiple small plugins vs. one big plugin. It's probably safe to say that there is a small amount of overhead in loading each plugin separately, but that effect is probably negligible.

Personally, I'd keep all of that functionality combined into a single plugin. You say they are for "different, likely unrelated, things", but I'm guessing that they all serve a larger purpose of handling the business logic. If you haven't already, you may want to check out the Business Logic template, which is designed as a starter plugin for exactly this type of scenario.