Battery discharge test by HKJ
Xpl v5 led test by TA
Looks like at 4.5 amps you would need 3.8v plus voltage losses so around 3.85 to 3.9v from the battery. Looking at the 5 amp discharge curve the GA cell would be close to that from voltage sag right from the start of the discharge if fully charged. Your basically running direct drive with that cell.
Try charging the cell up completly then as soon as it finished charging, try it in the light. It should be brighter for a few seconds reading a higher current. Also make sure your using a heavy guage wire to test with. They are also voltage losses in the tailcap current test wire. The bigger the wire the less loss. If you had a way to measure the light it be pretty simple to see. Eyes are not a real good at judging lumens.