It's hard to put exact numbers on such things - longboard in displacement mode, quality of the wind, difference between gusts and lulls, etc.
If you sail inland with flukey/gusty winds and you're heavy, then there's a chance that a displacement hull will bring you much more TOW and enjoyment. Yes the Formula will plane earlier given enough wind, but if it doesn't plane than it's a royal pain as you already know. Try before you buy, find/borrow an old longboard, or even better, Div2 or a Serenity. As I've said, it's not the numbers, but the general feel that's so much different.