@Kenshin, our policy for adding new sub forums is (usual, unless we think it's strategy wise different) is that when the parent forum is becoming too active. I understand you've might seen inconsistent behavior from us the last year but that's mainly because due to the change in ownership we've been a bit searching for the new way to go or that we've partnered with companies that link from their website to that particular forum.
What we normally do is split the parent forum in multiple sub forum if there activity is there. While instinctively you might think that opening a new forum would attract new people our experience is that it isn't the case.
Given the fact that we have very popular SSD reviews, I think there is room for improvement on the activity level of the SSD forum, this is currently my top priority and I have some ideas to stimulate that activity. However in it's current state splitting it would probably not contribute to it's activity level.
We've found that people usually post in forums where there is some activity and stay away from empty, inactive forums. So it's a bit of a snowball, once a forum gets active, more people jump in and once the ball gets too big we split it in smaller ones.
So my goal is certainly to create the subforums you've proposed but they'll like get there once we get the SSD forum more active than now. I hope I've explained it properly this way and thanks a lot for your suggestion!!!