Slightly elevated BUN and creatine is normal with rotten teeth, and normal in some greys regardless. Some greys just have bad mouths. Mizzy had a dental in February and I looked in her mouth the other day, and went "phooey". She will be an every 6 month girl.
No I wouldn't worry about food until you do blood work after the dental. Find out exactly what the values were.
Rotten teeth can also affect the heart and since they can do well without teeth, I'd take out the bad ones. I'd fight to leave the canines.
How old is she?