I have run a few chrony shots for you with the springs i have with my Delrin/Stainless guides/Tophats
Protek power pod spring and their own nylon guide and their 12.5 gram weight. 438 fps Avg.
(I suppose within 15 fps of the mentioned 455 fps in the AGW article and pellet dependent ....fair enough)
Protek spring but with the crap Nylon guide chucked out (way too short and tight) replaced with my own precision stainless steel slide fitting guide and 18 gram top hat and 2mm increased pre load 487 fps
Most power set up.
P18 Konceptus spring and resized version of above stainless guide and top hat (16.8 Grams)
462 fps
This is a slightly weaker but longer spring than the Proteks spring but very well made and finished for very little cost.
P18 spring but precision slide fit Delrin guide and small location top hat 435 fps
Most accurate set up.
Proteks spring but using delrin guide and top hat as above ...im working on it ...
All shots based on 3 shot string avg with Falcon pellets...