I can give you half an answer and speculation on the other half.

Given that radioisotopes decay with a halflife pattern, I would expect that you'd see a 16.6% drop in capacity every 23 years. So something like:

    1977: 100% (470 W)
    2000: 83.4% (392 W)
    2023: 69.6% (327 W)

To get a full answer, you'd need to know how the thermocouples degrade. Under the (completely unsubstantiated) assumption that they degrade linearly, that's about a 20% efficiency degradation per 23 years:

    1977: 100% isotope * 100% coupling efficiency -> 470 W (100% of capacity)
    2000: 83.4% isotope * 80% coupling efficiency -> 314 W (67% of capacity)
    2023: 69.6% isotope * 60% coupling efficiency -> 196 W (41% of capacity)

I suspect the thermocouple efficiency degradation is very much non-linear -- or rather, I have no reason to actually expect it to be linear -- so the second half of the above is a total SWAG, but maybe gives you a rough idea :)