Regarding the GP, I use them to and as far the test have shown the following:
1 train with 6 full railcars plus 2 engines, meaning 2 power function motors powered by one battery box with 6 x 800mAh GP recyko+ last around 2 hours.
Now the time with the official LEGO set, 60051, is around 5-6 hours, 1 power function motor with the same 6 rechargeable recyko+.