Are you doing 7% or 3%? If you're doing 7%, you should definitely try 3% first.
And as far as PFTs dropping, the same thing happens with TOBI so I wouldn't be so concerned about that as the are both lung irritants. Coughing as a result of HTS is also a GREAT thing and I wish it made me cough even more quite frankly. But obviously if you're spasming and can't catch your breath, something needs to be addressed. One thing is for sure, not all drugs work for all CFers, it's just a matter of finding what works for you.
Ronnie