If you draw a 50# bow @28" with a 500 grain arrow
and then draw the same brand of bow that is 50#
@30" with the same 500 grain arrow, how much
difference in speed would you see?  THANKS BOB

It is about 4% per inch if I am not mistaken.  I have never been wrong before though...  

Sounds about right.  The two extra inches of draw turns your 50# bow into a 55#-56# bow.

Bob, you should see right around 8-10 fps difference in arrow speed with PRECISELY the same weight of arrow expressed in grains per pound of draw.  That's because you store more energy with a bow that's 50# @ 30" versus a bow of the same design that's 50# @ 28".  Again, none of this comparison is meaningful unless precision is used to measure each bows' draw weight and each arrows' mass weight and each draw length.  But if precisely done that should be the difference reflecting the increased stored energy of the longer draw.

Take a look at this page describing energy storage - maybe it will help:   http://acsbows.com/storedenergy.html