If you are comparing a "fresh" install WEI with a later one, yes the drop is reasonable. Some of the diff could be accounted for in the percentage of fill and Number of blocks marked used and not yet cleared.
If you see NO day-to-day difference in performance - ignore it.
I also have two 120 gig Agility III's.
I only recommend them if the Price is right - Well below say the Curcial m4, Samsung 830s.
Reason. While they are label SATA III they do not perform any beter on SATA III than when used on SATA II. Reason I dub it a Sata II SSD dressed as sata III.