Here ya go....all GNs in feet @ ISO 100 @ 50mm
Sigma EF500 - GN 132
Sunpak PZ40X - GN 102
Metz 54MZ-4 - GN 132
Nikon SB-800 - GN 138
Now....taking into accout the slight differences between manufacturers testing methods, and design differences in the concentrating lenses, they are remarkably close. Truth be told, there's only just so much capacitor power you can put in a shoe mount flash, and these figures aren't but about 1/3 stop different from a 20+ year old Vivitar 283 (GN 120).
Don't worry about the power, concentrate on ease of use, features and cost vs. reputation.