You really can't get a single, quantifiable answer for any of the staff positions. Same with a lot on BB. Largely, it remains speculation. There is some phenomenally good and educated speculation out there on different things (in particular, the impact of trainers... ), but for the most part: it remains in the realm of guesswork.
For each of my staff, I have L6 (each with a specialty that benefits my team at this particular time). And if I can upgrade at a reasonable cost to L7, I will (in particular, a trainer!).
http://with-malice.com/ - The half-crazed ramblings of a Lakers fanatic in Japan