My guess is that the number is probably greater than 5%. You will find many on this site who do not like Tobi. It can cause SOB...drop in PFT's. I was on it for a long time and was always annoyed by it and when I started AZLI I dropped it. I think that it CAN cause SOME side effects and tightness; but, I think you have to look at how extreme the effects are and if they are worth continuing on it.
For ME, the effects were too extreme to want to continue and there are some that find that. Some people with a higher FEV1 may be able to afford a little tightness and slight FEV1 drop; but I was not one of them. It was too harsh for me. AZLI DOES NOT do that to me at all!