Like +1 multi and -10% AS? Some builds benefit (maybe not enough?), some don't (maybe too many?) but it's not clear BiS for everyone?
At an eyeball it's more like -5 to +15 on your arbitrary scale, IMO. That seems like a fine design space to me, but YMMV. I don't think it can be narrowed while still giving +1 multi (just moved), because the value of +1 multi is vastly different across different builds.
Changing it to -5%, for instance, would make it into a +0 to +20 IMO - which is bad.
Maybe I'm ignoring no-crit proc builds, for which it's just a negative even at -1% AS. But for those it seems like a fairly clear skip, same as Improved Crit, so that's ok to me.