NMK - very informative measurements on the springs. The Stock springs look like duel or triple rate springs. In working on dirt bike suspension for years, duel rates sound nice in theory but rarely work as planned. I prefer to revalve and keep a straight spring rate.
From the sound of it, over time your rig got heavier or your springs seem to have worn out. Good steel springs rarely wear out (so much for the stock ones). When the preload ring is cranked down on the stock springs to give more ride height, it can create dead coils and limit travel and nullify the reason for duel rate springs. The ones you installed look great and I bet they feel much more compliant (unfortunately we don't know what the valving looks like inside - I will attempt to open mine up this winter to see if I can "Fix" the insides). If an internal fix is discovered it would cost far less to revalve and respring then to buy new from Elka or Fox, etc.
I found this formula online to determine the spring rate for your springs (if you want to do the math). It's longer then the one I use but I'm at the office right now (and should be working but...) and can't get to mine. Always good to have a spring rate calculator formula around. Never know when you may need one.
Coil Spring Rate Formula:
11,250,000 - Torsional Modules For Steel (Constant)
CSPWD - Coil Spring Wire Diameter (How thick is the wire?)
8 - Constant
NOAC - Number Of Active Coils (Coils that are free to move. Not coils seated. It may be that 1/2 of the top and bottom coil is seated causing a spring with 8 coils to have the distance of 7 free.)
CMD - Coil Mean Diameter (The diameter from center to center of the coil. Measure the diameter of the coil from wire center to wire center. If you know the coil is .5 inches thick and the outside diameter of the coil spring is 3 inches, then the Coil Mean Diameter would be 2.5 inches)
(11,250,000 x (CSWD x 4))
(8 x NOAC x (CMD x 3))