Hi Jon Jones,
I can speak only as a larger vehicle owner (33 seater) - this has not really impacted me much - remember, you set a driver a target. How he achieves it is really up to him.
Matatus implement a similar *read kienyeji* airline/hotels Revenue management system. During peak, they will push up pricing and drop the rates off peak. The drivers know how to deal around this.
As an owner, only deal with the driver and my expectation is net daily (Allowance accepted for the occasional police/city council). When fuel prices are up, I gain as the net does not change. When fuel prices are down, he gains.