I would think that if you have lower or stiffer suspension then the bias valve might not work correctly anyway since its linked to the amount of lift in the rear suspension under braking?
If you had luggage in the boot and people in the back, you have lots more grip available from the back wheels, therefore they can provide more braking. The body to axle gap doesn't open up because of the extra weight. More pressure is sent to the back. Assuming of course the valve isn't seized!
The 1.3 SC I had didn't have the valve, and rarely had anything else in it but me. In the wet I could back it into corners rally cross style by being quite aggresive on the brakes.
Lowering the car would alter the range the valve was working in. Stiffer springs would reduce pitch making it less like to unload the rears anyway. Probably best without it.
How many Suds still carrying luggage and passengers in the back?
Really miss not having a Sud. Problem was spent a fortune on my Ti, and the bodywork still fell of it. Ran the 1.3SC for 2 years (around about 1999-2000). Put 35,000 miles on a car that had only done 35,000 prior to that. Was a great drive, but 2 Scottish winters killed the bodywork unfortunately.
So much nicer to drive than todays fat, bloated offerings. Think I should go buy a Lotus.