My pole vault accident.

[Barefoot]

Gripping down will "stiffen the pole" and might lengthen the radius to give you a better upward angle for your swing... ie more lift. Now if you take the same grip that you are blowing through on the 14' pole to the 15' pole you might find that it is Goldilocks just right... Just make smaller grip adjustments when moving poles than you have in the past. Just cause a pole is longer doesn't mean you have to raise your grip, as long as you are within the preferred grip zone.

[Barefoot]

If you get the chance... read DJ's post about take-off angles... viewtopic.php?f=37&t=19924#p142355
This might help clarify why you are over bending the 170 from 6 steps. The answer is not always the pole, but sometimes how you use it.