MacCready theory optimises cross-country speed over a closed course with the assumption that wind speed and direction are constant throughout the course and independent of altitude. These assumptions are approximately true most of the time, so departing from MacCready speed has little benefit. However, if there is a leg where this is not true, the optimum airspeed may be different. A downwind dash would be a special case of this.

I did some numerical simulations some years ago for the case where one is flying to a turn point against a strong wind and found a faster speed into wind followed by a slower speed after the turn was marginally better. Clearly, if your computer tells you to fly at 60 knots against a 60 knot headwind, you are not going anywhere! Another issue is that you have to maintain sufficient altitude to connect with the next thermal. This may require trading off speed for altitude and experienced cross-country pilots have their own rules for dealing with these circumstances. Flight computers and associated software are imperfect at dealing with these more complex issues which are up to the pilot to solve.

Mike