Ultra Racing has a range of chassis braces, some look like they might be good others not so much.
 
The fender brace and under body braces are the ones I'm interested in. Do you think would these actually do much?
 
Full range and prices here:
http://ultraracingaustral...tegory&path=60_153

I have the full front one they make and its rubbish and is ill fitting. It braces to the lower radiator support which makes no sense (a lot of these cheap braces just find areas under the car that look good i believe)
 
I have driven a car back to back with a different brand of the behind guard braces and did not notice any difference personally.
 
Tower braces obviously make a difference, but the cars already have a brace at the rear (rls4-964p) stock. The one bracing the rear trailing arm bracket to the crossmember seems odd, but i have never seen one either so cannot comment.

Pat uses a front underbrace which looks more effective than these ones. The front guard brace is used by EVO65 to great effect. Not sure on the rear underbrace having a cross brace under the car and the one above the engine should be ok. Would need to take a closer look at the braces fitted to a car before making a real opinion. 
I noticed a lot of improvement when I braced from the cage to the subframe in-front of the strut and a big difference when installing the cross brace at the rear. I have done very little up front and it is 'slower' than the rear of the car. There is no chassis delay in the rear but noticeable up front. Its seam welded and has the cage mounted to the tower but lacks cross bracing. This is not noticed until you are getting to and over the limit of the chassis. Well, in my experience that the case. Doesn't mean its right, just how I have seen it.

I have the mid front underbody brace on the spyder and very happy with it noticed the difference straight away. And perfect fit.

I think any bracing that is mounted at appropriate points is worth considering and the effects are noticeable as pete said.