Sounds like more then just asthma to me!!
Respiratory illnesses and masses can cause breathing problems and pain.
Has the kitty been xray'd?
I've been down this road with my dog who has since passed.
Go back to your vet and demand xrays or go to an emergency animal clinic...they give the best work ups as they have specialized docs on staff.
Good luck and keep us posted
(hi LuckyRescue...looks like I'm following you around haha)
Cats only have nine lives because they stole them from dogs!Teehee