I just did my first attack of a full 6 gym (Blissey, Snorlax, Dragonite, Exegguter, Steelix, Umbreon - a pretty tough combination). All 6 were fully motivated.
I cleared the gym with my first round attack, but was really surprised at the low level of reduction in motivation. Also there was no bonus at the end for a clear run.
Looks like a fully motivated level 6 gym will take as long to take down as an old version level 10 gym.
We really need to know the formulae for the reduction in motivation.