Wheel upgrades

Significant increase in speed?  Depends on your definition of significant.  I have an aero wheelset, a training wheelset and a climbing wheelset.  I don't think the aero set offers a significant increase in speed, but over all offers "some" increase.  I personally haven't measured the differences.  I've read tons of literature on this topic and from what I've heard there are several other cheaper improvements you can make that make a bigger difference in speed increases, for example an aero helmet. 

Comparitavely aero wheels make less of a speed improvement as other things, like bike position, fitness level, close-fitting jersey, helmet etc.  But if you have the $$$$ to spend why not buy a set of race wheels.  There's worse things to spend money on.

For specifics in speed increases I'd visit the HED web site.  They've studied aero benefits for many years.