If we're going to do a size category change, it should really be associated with taking the solid form rather than with the dust talons attack.
I'm still thinking the size change is unnecessary, and needlessly complicates things. The belker doesn't change size to attack, nor do most other creatures that attack by engulfing a part of the body.
I'm happy with leaving it the same szie. Are these abilties wrapped up in that case?
The Dust Devil can travel at will from the Prime Material plane to the ethereal plane, from where it makes its way back to either the elemental plane of earth or the elemental plane of air. However, becoming ethereal takes a full turn for the Dust Devil to accomplish, and the creature can be attacked while this transformation is in process. A Dust Devil which is reduced to 10 percent or less of its original hit points will automatically attempt to escape to the ethereal plane, or it may choose to attempt to escape earlier than that if it judges its adversaries to be too strong. A Dust Devil regains all its lost hit points, back to its original number, upon reaching one of the elemental planes.
Updated.
I like the revised dust form and really like the ethereal shift. I'm not fond of "dust talons", though, as it didn't actually attack the victim's inside. It was more of an asphyxiation. Let's try to retain that, to help differentiate it from the belker.
Something like...
Asphyxiate (Ex): A dust devil in solid form (see dust form, below) can engulf opponents by moving on top of them. It fills the air around one Medium or smaller opponent without provoking an attack of opportunity. The target must succeed on a DC 13 Fortitude save or inhale part of the creature. The save DC is Constitution-based. The thick, choking dust inside the victim deals 1d10+2 points of damage per round. An affected creature can attempt another Fortitude save each subsequent round to cough out the semivaporous menace.
We could take it a step further and include the effects of choking smoke from the pyrotechnics spell (-4 penalties to Strength and Dexterity).
Good point! Updated.That'd be OK by me, including the Str and Dex penalty.
However, if it's an asphyxiation attack I'd change the damage back to 3d4 since it doesn't make must sense to include a Strength bonus, doubled or otherwise.