General

Idk why everyone aims for 20% boss, doesn't seem worth considering there's 50% buff duration. Especially now with "free" buff freezers! Never rebuff, oh the good life.

i got 50% buff duration for my nl i got dsi so am keeping it cause thas helping me training/bossing all my buff duration are now 6-7 mins MW 27min the time u spend to recast all ur buffs decrease ur dps

Idk if it helps too much, but I have 30 att on my BM right now.

20% boss on my first line and other 2 are crappy atm. no idea what i should aim for

I prefer that % boss line for classes that use hurricane like skills. It's useful for your mob skills I suppose.

I personally have a boss line, item drop line, and my last line before I ran out of honor exp was only 10% buff duration. I don't even know what's the best I could get on a third line...

it's pretty eh for mercedes b/c if you have too much attack speed chaining skills becomes really annoying and is practically broken (e.g. rune of swiftness) but since it's only +1 it shouldn't be that bad. If you prefer it like that you may as well just use MPE pot
the benefit of having short startup on ishtar is pretty negligible too

It affects all skills except rapid Fire skills (in your case - ishtars ring). It will only help rapid fire skills with start up delays. However, if you train I would suggest keeping it as it is useful for all other skills and is a really rare line.
I'm a bowmaster (switched yesterday) and I have attack speed +1 even though most of my attacks are rapid fire. I do plan on keeping it as well (Mainly because I don't think I can get better than my 3 current lines).