I have not many questions to ask or answers to give on the EEVBlog Forum, so i usually visit single categories and check the first page for new content to see if something interesting is in for me, so if there is not enough activity per category, i would stay away. If too many other users do that too, topics would not get answered.
So imho the rate of posts per day is more important than absolute post count. I mean it is the active users that make the forum work, things should be oriented toward their use case (survey appreciated). And of course newbies need to be able to navigate to the right place, so regular users and new users need to be able to keep it working. A lot depends on the category description.
Other options to adding categories would be to generally increase quality of content (not saying it is bad..., it is another method to escape the clutter), by implementing a second order information upcycling like FAQ-Lists (!!) or a Wiki (imho horrible to moderate and organize). The search is of course very important and topics that do not name the subject or the right keywords are a lost case in whatever category they are in... never gonna find that old thread with the important information without the keywords stated in it. Not all threads require naming a make and model, but well, this is the Products->Test Equipment category, most probably do and good threads contain that information and are therefore found more often.
I usually favor these over other methods, because people that complain about others cluttering the forum can be sent that way to ... maintain an FAQ or do a good writeup on a subject that can be pinned or linked in a fixed place.
Category, age or absolute post count would not matter as long as the search can find it, but the content needs to be filled in by active users that visit these areas.
Another effect to consider: the more context is provided by a category a certain content is in, the more of this context is left out in the actual thread by whoever writes it, making this context unavailable as keywords.
Forums are complicated... and complaining is easy. I´d wait till there are new posts to fill the first page within a day before splitting into subcategories.