The best justification for splitting the engine forums is that people tend to be very self-centered when posting. They post related to their specific car and often don't provide the relevant details.
When I first got my MY00, I remember reading many threads ultimately related to MAF issues where the poster never specified that he was talking about an MY99. That's one stupid example, but I can easily imagine posts in the Turbo forum where a WRX owner just ASSUMES everyone knows he's talking about a WRX and not a turbo 2.5 or 2.2. Likewise, there are going to be a lot of upgrades and kits that are specific to the WRX and won't do a damn bit of good to anyone else. I can see it now:
"Announcing cool, new stage 7 boost control, ECU, downpipe, wastegate kit."
"Sounds great! Send me one for my car!"
"Ummm, forgot to mention, WRX only."
Frankly, I believe the community will get MORE pissed off and divided if this sort of thing happens. Let all Subaru owners come together in area of universal interest, but when I need tech help, I don't really need to wade through a bunch of WRX owners who have nothing relevant to say.
And guess what? When there ARE technical issues of common interest, when there IS a WRX part that might be useful on my car, nothing stops me from going over to the WRX forum to check it out.
I see no downside to splitting the forums.