Sedation can be safe, but I wouldn't trust a groomer to do it. I know you said you'll be taking your cat to the vet, so I would assume your cat would be closely monitored while sedated, and the sedative they are using is meant for cats and properly measured.
We've shaved quite a few cats (and some dogs) under sedation, as well as doing "short" tail trims on sedated dogs. We haven't had any problems.
I should ask - why do you have your cat groomed? If its just for looks, I wouldn't recommend putting him through the risk associated with sedation or anesthesia (even if the risk is low) just for looks. If he's an older cat who has stopped grooming and is severely matted, then you've got a good reason.
Also, how "bad" of a job did they do last time? If he's a well behaved cat who just gets a little squirmy, I'd take whatever you can get without sedation. You can also try an actual cat groomer or in home groomer who specializes in shaving cats. I don't know how your vet's office works, but if its anything like ours, we get the job done and it serves its purpose... but its not always pretty
So in short, sedation is safe when done properly but it might not be necessary. You'll have to weigh the pros and cons, and see if the risks are worth it to your loving companion, just to make him look nice.