Cool stuff, neat to hear. LOL I tried the same idea just after I posted, adding some preload... I used a tiny dremelled off piece of allen key, but I couldn't get it to work without coil bind, there's not much space between them. I'll try it at your suggested 2 coil spacing and see what happens, thanks.
The Gre Tan spring does just seem to work by being longer - it's intended as a fix for holes drilled too deep on Rem 700 SAKO extractor conversions. It looks like the same wire diameter to me, meaning the same spring rate; ie, no "stronger" in spring terms. Adding preload doesn't make the spring rate higher in any way, but does make the force needed to get the spring moving higher in the first place. So if it's say (and I'm just making this up obviously) a 4 oz per 1/32" spring rate, compressing it 2/32" will mean it exerts 8 oz of force against the cartridge rim, rather than say 2 oz if it's compressed 1/64 normally. For our purposes, that's exactly what we want, more stiction to the bolt face.
Short of going to a higher rate spring, adding preload is the only simple solution I see here. A more extreme one would be to bore the spring hole out slightly and use a bigger hammer as it were.
Thanks kiki, I'm going to revisit it today again.