Had a steeda bar in my 2006.... Not going to lie I didn't notice a single bit of difference with it, then again, I never had mine welded in. Looked fantastic though, caught a lot of attention at car shows and whatnot.
1. Rear shock tower brace offers no gains in terms of rear suspension geometry. All of your rear suspension geometry is actually calculated through control arms, or panhard bar/watts link. Any deflection that is going through the rear tub of the car has very little effect on how the car handles and puts the power down.
2. I'm not sure what kind of bushings you'd use on those, but if it's stock rubber bushings, then your whole brace is rendered useless, due to massive amount of deflection in those rubber pieces. Also, there is no triangulation between the the shock brace and the rest of the floor pan, so even if there was any load on the rear part of the car, you'd just see the rear towers walk along with the chassis like a weak square frame.
3. Shocks towers carry absolutely no load unless you are running coil over conversion. There is very little deflection caused by loading on the rear suspension
Just to answer the question if you wanted to DIY a brace. I would find a car that is similar width buy the bar for that and either drill holes in the mounting plate to match up with yours or make a new mounting plate. If you can make the mounting plate you can make the whole brace though
The steeda rear stb HAS to be welded in. http://www.americanmuscle.com/steeda-2005-rear-shocktower-brace.html Mine is tig welded into place. If it's not welded, it's not doing anything,since the hole in the top of the brace's bracket is large. The bracket's two lips are folded down 90 deg..which is where they get tig welded, onto the sides of the rear shock tower's. The sheet metal between the rear struts is thin enough as is.
On a similar note, steeda now makes a rear X brace that does not require a rear seat delete. The bottom corner's of the X are welded to the rear shock tower's. The tops corner's each have 3 x holes. 2 holes are used for bolts, the 3rd hole is for welding. AM tells me that the steeda rear STB and the new X brace can be used simultaneously. New X brace is made from 1 3/8" OD chromolly tubing, .083" wall. http://www.americanmuscle.com/steeda-xbrace-0514coupe.html
Shipping weight is 14 lbs. Net weight is 10 lbs.