I would just like to report that drop rates do seem to have lowered from the rat Lord boss down to killer siafu from when I last had an account at that level (about 6 months ago) perhaps this is coincidence or maybe they were lowered due to many special quests that allowed more players to one hit kill these bosses?
however the drop rate seems to drastically improve on mordoraf and above so just be patient until you reach those bosses
for comparison I can use about 1000 energy on rat Lord and lower and gain maybe 400-600 hp, however using the same amount of energy on mordoraf and above usually results in anywhere from 800-1200 hp (one hit killing)