Well it's not exactly 1500 because of buffs and aa's that increase hate by a percentage, they apply after the 1500 cap. If you are seeing its higher at higher levels, its probably because of this. Either way, heals are not a significant amount of hate. With no hate mods a heal moved my aggro on my test subject by about 3-4%. FOD, which is 20k hate, moved it by about 70%. the heal does about 10% with hate mods on. My source for 1500 comes from a certain project on github, that is apparently against the rules to mention because it edits it out if you try to post it. I'm fairly certain they've tested it.