If it only were that simple
Horses for courses I think. Mach tuck became a big thing when the first Lears came out and folk were including secret overspeed warning cut out switches and hence learning about Mach tuck first hand, it lead to a number of accidents and incidents, being a T tail personally wouldn't expect it to be much influenced by down wash.In any event, because the contribution of down wash on the tail is so rarely mentioned I would assume its contribution is minimal.