ok, I did run a test by setting an to run the service as myself. I'm in the administrators group on the server. Running a test job from ESP that executes SET to dump the environment shows me as the user (I ommited the USER statement from the job definition). I then added USER statement using a service account and it failed. The CSF showed the following:
CCode Job Status
1314 Insufficient privileges
So it certainly appears you are limited when using a specifc ID to run the agent.