BBK: Really if you can just find one to reduce rotational mass via 2 peice rotors that'd be sweet but I think you have something wrong in your system. I'll try to remember to take a vid of me hard stopping. SS lines, dimpled slotted rotors, and good pads and I stop real nicely. I have notice the lack of ABS "kick" I was used to from my Impreza but have felt it.
Spring rate: To note check this http://www.subaru-svx.net/forum/showthread.php?t=45157
Also TomSVX on there is on NASIOC too and I think he user name is tom5spdsvx I'd take the time to ask him what he knows. Or check www.allwelldrive.net
Power Adders: Again ask Tom he's the only one making a real CAI option thanks to the twin throtle bodies. the rest is just general stuff like normal really. ECUtune.com for internals and ecu work. Exhaust everyone does different stuff, mostly custom one-ofs though.
Motor Mounts: People have been using modified Group-N mounts. Basicly shave to fit.
I can see you guys really want to find out a lot on your own but really please check in at www.subaru-svx.net
Lots of people have invested lots of time and money into these for personal gains and share their findings. Just trying to help you to help us(SVX owners)