You have certainly come a long road with this disease. With BUN around 100 and Creatinine around 5 for the past 2 years, you are very lucky to have a very resilient kitty that can withstand those kidney levels.
Since these numbers are unchanged after all this time, it is reasonable to assume that her severe level of anemia is what may be contributing to her being so weak. Epogen is a good medication, as it provides the hormone stimulation for the bone marrow to begin making red blood cells - primarily the job of the kidneys that clearly cannot provide that function at this point. 8.5% is a very severe anemia, and one treatment of Epogen is not likely to change that very soon. My suggestion would be to give her a blood transfusion to stabilize that anemia, which will likely stand to make her feel better, while the epogen has time to work.
A first time blood transfusion for a cat does not require a crossmatch, so you can simply get donor blood in infuse it into the IV. A donor cat must be at least 11 pounds in size. In my clinic, medical team members and doctors typically provide their owner cats for use as donors...it really is no big deal for the donor and recipient, and may go a long way toward buying your kitty some time.
Please feel free to discuss my recommendations with your veterinarian.
Thank you - one last question my vet I can tell does not want to do the transfusion- he said I could go to the emerg vet and it would be 500-800 does this sound right? I have 2 cats who haven't been typed but are big and healthy. Thanks!