I did this test today too. I had two low power springs (11lbs/12lbs) and decided to just do the 11lb spring:
Underwood 180gr FMJ (Reloads) out of the Match with a 22lb spring as a baseline: 1269, 1279, 1246, 1262, 1294, 1270. Average = 1270 FPS/ 644.53 LBS.
Underwood 180gr FMJ (Reloads) out of the Match with a 11lb spring: 1263, 1241, 1272, 1249, 1229, 1276, 1246, 1272, 1231. Average = 1253.22 FPS/ 627.62 LBS.
Pretty much debunked the theory in a Match. Possibly different in a Glock.
I still wanted to do worse case so did Underwood 155's.
22lb spring: 1568, 1598, 1553. Average = 1573 FPS/ 851.44 LBS.
11lb spring: 1618, 1629, 1599. Average = 1615.33 FPS/ 897.88 LBS.
Yea. Blew this one out of the water it appears.