[smuzz]

(Warning: Maths = ON)

Gazz - I think those calculations you've got there are speed calculations (kind-of) as opposed to thrust calculations.
You've got ...

rpm per volt x number of cells x pitch in inches

... if you modify it a little ...

rpm per volt x number of cells x 3.7 volts x pitch in inches = speed in inches per minute
rpm per volt x number of cells x 3.7 volts x pitch in inches x 0.000947 = speed in miles per hour

So Hacker 50-14S:- 425kv x 6S x 3.7 x 12 x 0.000947 = 107.2 miles per hour

This calculation is over-optimistic though because the 425kV figure is only for no-load (as soon as you put a prop on, the rate of increase in rpm per volt drops below 425 and it drops differing amounts for different props), the cell voltage drops anyway and the prop never actually acquires its true pitch speed because of the drag of the rest of the plane, amongst other things.

But, your comparison formulas are not bad as a guesstimate (the fact you haven't got the cell voltage in there doesn't matter if you're doing a comparison) - probably much more useful to get the percentage differences as you're doing, than trying to estimate absolute speeds or thrusts or whatevers ... Sorry mate - I couldn't help myself.
(Maths = OFF)

All that crap aside - I think Kieran is probably right too!