Mageslayer has +45 damage too.
So if you want to compare..
+22.5 dmage
+12.5% IAS
+6% life steal for 4k
vs
+30 damage
10% damage for 3.3k
It's not just that though, it's item spots too. More expensive items shouldn't(and usually don't) give as much benefit for their cost, they just give high benefit per slot.
And it's not just that, but IAS stacking. You get mageslayer and ashkandor, and that's +50% damage. +50% is fine when you're doing base damage, but you have +dmg items being MULTIPLIED instead of added as well as increased procs(like from poisoned dagger, wyrmskin)
IAS is the most powerful offensive buff. If you have 100 damage, +25% IAS is the same as +25 damage until you add +dmg items, % chance on hit items, crit more, and so on.
So yes. Mageslayer would still be worth 8k with 15% IAS.