That may be true in gneral, but I believe in most races I have seen, such as the Adirondack 90 miler, the guideboat rules include "Seats must be fixed, no rolling seats are allowed."A sliding seat will also increase the performance.
As opposed to rules for canoes , C4 and voyageur canoes may have gunwale-to-gunwale rolling mid seats to change paddling sides during huts.