Obviously, you can make the car too stiff, but you can go too soft as well.The kit yellow springs are too soft IMO. At the moment I am running 16 or 18 pounds in the front, and 14 or 16 pounds in the rear - I don't think that's too stiff, but I agree if I ran "carpet" style springs it would be too stiff.
I have only had this car for 4 meetings so I'm still learning with it, but atm it is working better when it is stiffer.
Whether or not there is more stability is down to the driver and how they like the car to feel IMO. I don't like any car (off-road or on-road) to body roll - as it feels very slow and has poor reactions.
I try and set my cars up stiff so that they respond VERY quickly. They are not stable at all like this (opposite to your findings), but in fact VERY difficult to drive.
In the end lap times are all I go by - and I get better lap times when the car is stiffer even though it is much harder for me to drive. It could be just my driving style but that's what I've found so far.
Interestingly, the tuning guides from XRay, XXXMain, Losi, & various internet sources never agree about these things either - so I'm not sure if there is a definitive answer when it comes to the way these adjustments work.
I am currently using the JRXS set-up guide as it's findings are similar to mine.
My roll-centres are currently:
FRONT: 3mm under inside
REAR: 1mm under inside, 2.5mm under outside
BLOCKS: std - no shims
My comment are always based on asphalt or concrete only as I have no experience racing on carpet.