I went through the same thing earlier this year.
You may find this post in my P52 thread useful - it compares the Nikon P510 with my trusty Nikon 70-300mm
FWIW I am also considering the Canon SX50HS, both for the RAW capability and the hope the AF and V/F will be better than the P510.
If you are considering the P510;
the 1000mm FFE lensBad points;
the rear LCD being 910k dots and tiltable
in camera jpg correction of lens defects, etc.
the EVF is really naff; impossible to judge if focus or exposure are even in the ball park
In good light, with a static subject, the 1000mm beats the 450mm FFE of the 70-300mm on a Nikon crop bodies like ours.
I will be publishing some further comparison shots soon in that thread.