I figured it's been over a month now, and I should update in case anyone's following.
No more hacking/coughing after the one injection! I can save up my Benadryl supply :). It was well worth the $30 for the shot. Rudy the cat has not had a single episode in well over a month.
I did keep the litter changed to pine pellets, I buy the ones for the horse stalls at the feed store ($6 for a bag versus $10 for the cat version). It tracks horribly and turns into sawdust if the floor is damp, but no more litterbox smell either!
Hope everyone's cats are doing well :)