I'm not exactly certain of the numbers here, but its quite easy to make a macro which pops you into Hawk whenever you cast any instants or into Fox when you cast Steady/Cobra. There is some dps loss to be sure, but I'm fairly certain that that dps loss would be outweighed by the benefit of being in fox if you ever have to move, and you'll always have Hawk up when you cast an instant. Basically the macros would be:

#showtooltip spell_name
/cast !Aspect of the Hawk
/cast spell_name

for everything other than steady and cobra and then:

#showtoolip spell_name
/cast !Aspect of the Fox
/cast spell_name

for Steady Shot and Cobra Shot. This is very similar to what I use on my leveling hunter so that I never have to manually get into Hawk when I've been running around in Cheetah or what have you, although I don't have Fox yet. This is much simpler and faster than manually clicking in and out of Hawk and Fox, and while technically there is a dps loss I imagine that its more than made up by the facts that a) it demands no attention from you allowing you to focus on other aspects of the fight, which may very well improve your dps, and b) It takes no time from you, so you're never wasting any time switching aspects while you could be casting