I noticed this behaviour whilst levelling, but I only went back and had a proper look at it today.
This is using Rhok'Delar (2.9spd), Serpent's Swiftness and 15% quiver.
This gives 2.101 speed normally and 1.501 speed during rapid fire.
Ripped from my combatlog, edited to get rid of a lot of "spell not ready yet" and "mana drain" spam.
2/8 13:52:23.218 Wind Serpent gains Bestial Wrath.
2/8 13:52:23.234 You gain Blood Fury.
2/8 13:52:23.234 You gain Rapid Fire.
2/8 13:52:23.249 You gain Ferocity.
2/8 13:52:23.530 Your Arcane Shot hits Dr. Boom for 524 Arcane damage.
Run of the mill powerup macro
2/8 13:52:28.624 Your Steady Shot hits Dr. Boom for 423.
2/8 13:52:28.718 Your Auto Shot hits Dr. Boom for 372.
2/8 13:52:28.937 Your Auto Shot crits Dr. Boom for 853.
2/8 13:52:29.327 Your Auto Shot hits Dr. Boom for 360.
I'm writing this section off as slightly bizzare lag, because 3 autoshot hits and steady within 1 second is too odd for me to try and explain.
2/8 13:52:30.812 Your Steady Shot hits Dr. Boom for 379.
2/8 13:52:31.280 Your Auto Shot crits Dr. Boom for 876.
2/8 13:52:32.562 Your Steady Shot hits Dr. Boom for 410.
2/8 13:52:32.796 Your Auto Shot hits Dr. Boom for 396.
2/8 13:52:34.077 Your Steady Shot crits Dr. Boom for 942.
2/8 13:52:34.859 Your Auto Shot crits Dr. Boom for 875.
2/8 13:52:35.874 Your Steady Shot hits Dr. Boom for 378.
2/8 13:52:36.312 Your Auto Shot hits Dr. Boom for 355.
2/8 13:52:37.468 Your Steady Shot hits Dr. Boom for 381.
2/8 13:52:37.796 Your Auto Shot hits Dr. Boom for 347.
In this section, we see steady shots landing more or less every 1.5 seconds, and autoshots landing about 0.5 seconds after each, suggesting that the steady shot is taking less than 1sec to cast, so the auto has time to fire in the split second where I'm hammering steady but haven't hit it yet.
2/8 13:52:38.046 Blood Fury fades from you.
2/8 13:52:38.062 Rapid Fire fades from you.
2/8 13:52:38.062 Ferocity fades from you.
macro ends.
Essentially, the behaviour observed is that you can spam steady as fast as you can click, without ever clipping the autoshot, nor preventing it completely.
Does this suggest that if we can find a good weapon with a 2.1 speed (down to 1.52 base after swiftness and quiver) that one can fit steady and auto-shots together almost perfectly?
I tried this with the Maghari Light Recurve, but it refused to accept a quiver bonus or use arrows, and I've not yet made my way to a vendor to buy a low level 2.1 spd.
More generally, am I missing something, or does this disprove the theory that 2.8 is the ideal speed for BM hunters?
edited for spelling.
update:
Tested with 2.0 - 2.3 speed bows, don't see the same behaviour - I suspect that the 1.52 speed is too slow to allow the same behaviour as the fully hasted 2.9->1.501. a 16% haste quiver would take a 2.1 speed bow down to 1.508 which might be close enough to 1.5 for the noclip behaviour to occur.
Barring that maybe I'm just going crazy. Does anyone else see the same spam-friendly behaviour from 2.9 speed bows?