Well its not really about stiffness, just how well they control the oscillations of the spring. Having said that the originals are pretty crap/shortlived & as you have found out, easily replaced so I would just replace them unless you know they are quite new. Konis are probably best price/quality compromise, improves ride & handling. I've put them on the back - OE still on front.
I am guessing my standard ones are goosed.
I have some off my brothers old mini that are firmer than mine and his car only weighs 600kg.
Also when you push mine down they seem to spring back up quite quickly. Except the last inch or so that doesnt return.
Mini ones are hard and slow to push down and rise slowly on their own. Assuming these should be similar.