Are shadows from a light dome ever really soft?
I don't think so; even when they are set to soft shadow of 100. I don't see any difference in the shadows whatsoever. They are hard as can be.

Light Dome render times:
Reg. AA w/o soft shadows: 2 seconds - 3.26 million shadow rays
Reg. AA w/ soft shadows: 1 min. 48 sec. - 2.09 trillion shadow rays
9 RPP w/o soft shadows: 19 sec. - 2.80 billion shadow rays
9 RPP w/ soft shadows: 19 sec. - 2.80 billion shadow rays
But the softness setting does nothing to the final appearance. 0 softness looks identical to 100 softness. You can see that same number of shadow rays is used w/ premium soft shadows as premium w/o soft shadows. That is with both the lights and the render setting w/ or w/o soft shadows. Swithcing one or the other on or off makes no difference either.
Obviously there is something not working correctly w/ shadows and light domes (IBL too).
So the fastest by far is Reg. AA w/o soft shadows.
The slowest by far is Reg. AA w/ soft shadows.
But; there still isn't any difference in the appearance whether the dome lights are 'hard' or 'soft'.

IBL lights don't have soft shadows either. It is only when you turn the quality (equals # of IBL lights) up that the many overlapping edges make the shadows look soft. There is no setting for softness in the IBL system, so the times behaves as regular light domes using no soft shadows.
As far as I'm concerned, IBL in 6.1 and after is completely broken, or at least not usable in anything approaching reasonable time. To get a decent IBL effect, the quality must be cranked up, and then it takes FOREVER to render.

EDIT -6.0 IBL wasn't like that. They should have left it alone.

Friends don't let friends use booleans.