Vet recommended Hills Healthy Advantage

Have you looked at some IMHO higher than the SD quality foods with some dental care in them..

Nutro Natural Choice has dental care in it and is lower protein but still high I think 40% in the adult now

IMHO I would not feed the SD if you were willing to try something far Imho lower than Orijen ... most of the Grocery brands have the same ingrediants as the SD

It is a hot topic in the vet field today... I can only say Digestablity of the Protein IMHO is important ...

Have they had blood tests recently? If so, and their tests came back normal, I wouldn't change anything. There's a lot of debate about high protein and all that, but high-quality highly-digestible meat protein shouldn't be a problem. I'd have a lot more concerns about feeding a brand with corn gluten as a main protein source .

In my opinion, I'd stay far away from any of the S/D lines. I've worked at a vet as a technician, and I won't feed it to my own animals nor reccomend it to clients. It is basically high priced grocery store food. You could get the same quality ingredients and pay alot less going to the grocery store. That being said, I really like orijen and it is in my rotation for my own kitties. I rotate high quality brands to give a change and to make sure that they are getting all they need. My own cat was diagnosed with possible megacolon and had to have frequent stool extractions because she was on Science Diet which is what the clinic I worked at feeds. Well, I took her home and got her on a combination of wet and a mix of grain free and grain in dry and she is doing great. Hasn't had a bowel incident since October of 2009. I do give her miralax when I think she might be backing up, but she goes shortly after that. This is an example of why i think diet is so important.
Some other brands you might look into are Natures Variety. The praire line has grain, the instinct line is grain free. I currently use both. There are also Blue Wilderness, Wellness, Acana which is made by Orijen company and many others. My point being that, as long as your cats are young and healthy and don't have any preexisting illnesses or kidney issues, I don't see these foods causing a problem. Make sure you have plenty of water available as they will need to drink more. I give canned twice a day also as it adds moisture to the diet, and I think that all dry isn't the greatest. To combat tartar, I brush my cats teeth a couple times a week, and add a wax that lasts on their teeth. Dry food does not clean the teeth any more than crackers clean ours. I don't really believe in dental diets either as I haven't seen them work miracles. Certain animals will develop tartar faster than others and will need their teeth cleaned more often. I do find that brushing has helped alot in keeping tartar at bay. My dog has only had a cleaning twice in his life, and I just did it because he was already asleep for something else and not for the dental. He is ten and his teeth look great. He eats a mix of canned and dry as well.
I would stick to the orijen if it is working for you otherwise and take what the vet says with a grain of salt. My own boss will reccommend Hills as well, but then again it is what we sell, he will also reccommend the other brands if you ask his opinion, but hills seems to be a go to brand.