Many peoples perception of longboards come maybe like yourself, from many years ago with the good old 7.5 rig.
I agree that the sailing was not very exciting in less that 10k and a formula is above 10k.
But as I suggested, with todayís modern raceboards and suitable 9.5 or even bigger rigs, itís a whole different experience.
Even in sub-planning conditions these boards are fun. In 4 or 5 knots you can be hooked in, railed up, with board speeds above 10knots easily achieved.
They plane a bit later than formula boards, but often earlier than big slalom boards.