J.M.
I would ask your supervisor for a meeting.
At the meeting, or even meeting for coffee, bring up your workload.
"How do you feel I have been doing with _______(extra work)?" If supervisor has something positive to say about it, you can respond with "Great! I was wondering if I might be able to get that promotion or pay raise" (alter the words to your own style/agreement...)
When (if) the supervisor tries to put it off even longer, or refuses.. you can say "well, I feel that the stress of doing this extra work, with nothing to show for it, is detrimental to the work I AM being payed for... I would like to transition back to my paid workload."
or something along those lines... Be polite, professional, and FIRM. Let him know that you are only willing to do the work that you will be paid for.