The progress is almost 0. I once trained a capped player for like 12 weeks to squeeze one more up out - and quit because there was no (visible) progress.
On the other hand, if you have the training slot open anyway - you might get lucky and get a pop because your guy has high sublevels.
Größter Knecht aller Zeiten aka His Excellency aka President for Life aka Field Marshal Al Hadji aka Lord of All the Beasts of the Earth and Fishes of the Seas aka aka Conqueror of the Buzzerbeater Empire in Europe in General and Austria in Particular