How have levels of outgoing longwave and incoming shortwave radiation changed over the last 20-30 years?
Have we seen any changes in the flux associated with the recent "slowdown" in global warming?

There is no evidence for such a slowdown, nor for a change in the associated flux.  Atmospheric warming has not been significant in the past 15 years (nor can we significantly state that it hasn't warmed), ocean warming has continued.  As the oceans absorb the vast majority of the excess heat, this net flux probably has not significantly changed during the "slowdown".  Note that 15 years is in any case too short to state anything about global climate change.

As for your other questions: estimates of the flux have certainly changed over the past 20 years. but estimates vary quite a bit, so I don't think we can significantly state that there has been a change in the fluxes over the past 20 years.  Nor can we state the reverse; it appears further research is needed.

For an excellent and still somewhat recent overview article on the topic, see:


Trenberth et al., Earth's Global Energy Budget, BAMS 2009