I have run the Stewart LBS valve in the rear of my Cobra racecar with great success. It introduces a slight delay in the transmission of full pressure to the rear brakes by just a few milliseconds allowing the chassis to settle ever so slightly after the initial surge of pressure. All of the NASCAR cars have been using these for years. I think I recall that DPI has purchased the rights to the valve now.
Here's a pretty good article on it; Brake Line Shocker
On the Brake Guard - I have also run these successfully, however, they do seem to introduce a slightly spongy pedal. These have been around for more than 30 years now.. Basically it seems like these valves are just pressure dampers (much like air in the line - only controlled).. Worked well on my old Corvette racecar, but I had a real bad rotor problem on that car that would have benefitted more from proper ventilation.