There's actually a big following in NASIOC's OT forum... lots of former WRX owners switched over.
And the MS3 forum I was on back when I was considering one was Mazda 24/7... they've got an MS3 specific area:
http://www.mazdas247.com/forum/forumdisplay.php?f=142
That's really the only Mazda forum I've been on at all, so I don't know how it compares to the others. But I found it to be just a little below NASIOC's level for technical info, and relatively friendly... but I haven't been over there in like a year now.
Also, coming up with a line of MS3 intakes is a *great* idea. Those cars make like 30hp more with a good intake, and very little to no tuning is required as long as the MAF readings stay true. So intakes are hugely popular on the car, and the car itself is becoming a popular tuner platform.