Overspeed as I use it is wind assistance, not towing etc. In any event, my point about safety extends to both training and to the races themselves. If you are sub max all the time in training there is a significant jump between what your body is trained to accept and what it will be exposed to in races- that is where the risk comes in from the volume side, assuming you haven’t already built up a tolerance to meet speed in the past.
as for the combo stimulus, many groups use a training approach that replicates this (Speed, SE, off etc) so it might not be too big a stretch for them IF the first intensity stimulus is extremely limited in breadth (1 or 2 x flying 20m as the session for example)