With higher rep sets, I think the key is decreasing the rest period and not going to failure on every set. That way you get cumulative fatigue. These are the two ways I've found I grow: heavy weight taken to complete failure in the 6-10 rep range and lighter weight with short rest periods building up to a set taken to failure after several consecutive non-failure sets at rep ranges of 10-15.
This is 100% accurate. You want to optimize for all variables, this means you can't gain 20lbs without putting on some strength, and vica versa. There's extensive research showing that higher volume of non failure work> lower volume of failure. Ex: you're squatting 275. You could do 6 sets of 8 reps, or 10 sets of 6 reps, the latter being superior.