The gravity difference shouldn't be much, if any. Color in beer isn't linearly additive, so no, 5 pounds of C10 won't add the same color as a pound of C50. Ignoring the fact that you should pretty much never have 5 pounds of crystal in a 5 gallon batch, the flavor will be wildly different as well. The process that caramelizes the sugars changes the flavor of the grain drastically as well. Crystal malts can be fairly easily subbed out if they're close together in color (10L for 20L etc), you can't really do the same for grains of very different color.