Bars don't tell the story. Check the dBm numbers that the 6620L shows both with and without the booster. The real world test is to run speedtest.net (or similar program) on your computer multiple times and compare the results. You'll need to do half a dozen repeats of both in the booster and out of the booster to get averages, as the results can vary quite a bit even done back to back.
Testing often shows that when you are in a strong Verizon area the boosters don't increase the actual download and upload speeds near as much as they do when you are in an area with a weak Verizon signal.
It's my understanding that this has to do with the MiMo antennas on the 6620L and the single antenna on the boosters.
