I'd like to speak AS a cat groomer.
1st........we do NOT allow the owner to stay. And it has nothing to do with mistreatment of the animals. Animals are simply NOT as well behaved with the owner present, whether you care to believe that or not. And the presence of ANY owner will upset ALL the other animals being groomed at that time.
2nd........since I do not work at a vet's, we CANNOT sedate any animal. IF it is apparent that the cat (or dog) needs tranquilization, we tell the owner and they can chose to have their vet prescibe something or they can go elsewhere. (This only occurs about 2 % of the pets)
3rd.........the cat will most likely get a shampoo. It is NOT harmful to the cat in any way, but you will NOT get a nice looking end product without a bath. And since you will probably be paying $50 plus you might as well have the cat looking good.
4th..........The Petco in your area may or may not have "good" groomers. But then the private shops may or may not have "good" groomers. Try to get recommendations from pet owners. A LOT of groomers will not do cats. Grooming cats is DIFFICULT. And it is an acquired skill. NOT the same as doing a dog.
with that said.......I LOVE to do cats and I think a lion clip would do your cat a world of good. I have a shorthaired cat who is prone to hairballs, and has been SO good since I have been shavng him.