The beauty of this weapon is not so much the bonus as the ability to play songs that can only be played while wielding a brass instrument, like the Ervaj series and Denon's Dissension. Your group will thank you for being able to give them 80% haste without an enchanter at lvl 60.
OK Plugh, I will bite. How precisely does this weapon help a bard to haste her/his party. An instrument bonus does not in any way affect the haste or slow percentage on a song. As such, this instrument does nothing to make 80% haste possible.
However, that is not to say that a bard cannot haste a group to 80% or more at lvl60. For example the epic proc (55%) plus warsong of vah shir (25% stacking; ignores haste cap) would provide 80% haste. Adding composition of ervaj (10% stacking) would make this 90% but generally would not be worth it.
My apologies to non-bards for this post. I wouldn't ordinarily post something like this on an item but there is so much misinformation about I feel it only right to correct this idea.
Reed's song analysis spreadsheet (at least, the version I have) doesn't indicate any stacking exclusion between the two Ervaj haste songs, so I'd personally call it 85% (since I'm usually twisting four).
The important distinction to make here is that this mace allows the bard to haste the party to 80% while still himself fighting. The Ervaj series (well, pair) has been one of the least used groups of songs because they require Brass and do not provide enough haste to justify dropping weapons and sitting in the casters' corner. This weapon is clearly designed to address that shortcoming for the bard who likes to melee and group haste.