this is something i was trying to test, and i feel i have found an answer to satisfy myself.
i was having trouble with the bspec driver on the OPEL speedster event. i beat all the events except for one of them out of the 5. i needed to go faster, he wasnt doing it regardless of how i setup the tranny... softer tires helped alittle bit, but still no huge improvement.
so i went on a quest to raise my bspec skills. i had some pritty bad skills when i started... so i rased them alot, course skills, and machine skills, im not sure which one will have the effect, im guessing BOTH are needed, but am not certain.
but with 99 machine skill points, and around 80+ course points i was able to increase the speed. i didnt check at each incrament, so i cant say at what score will make an improvement, so its going to take some testing to do so, and im not doing it

no memory card yet

i hate not getting my fill.
it has to also do with the tune of the car too... so it seems, it has to be stable on the course like someone said above. when the bspec scores are higher they are more stable, but also a good tune will help ALOT!!!!! i cant stress that enough.... if he can corner very well he will go faster very well, that is with a higher bspec skill.
i havent got the bspec driver to a TOP speed, but he does improve, i havent got 100, 100, 100 yet, and i dont really plan to get that score, but i want to learn as much as i can.
some tips to take are to dissable the OVERTAKE option... this will make your drivers line less confusing and more stable. basically it will take the correct line, instead of going wherever to pass people.
i know this works because i tried it first hand... if anyone can prove me wrong then try it, but i have done proper testing and i can reproduce it if need be (dont think i have to tho).
i belive PUSH is usually faster but i usually keep him on FAST (4) after gaining first place... and if the turns are hard to turn on a certain course at a faster speed then i choose a lower number 3 or 2 after gaining first possition.
good luck guys... a good range of bspec skills to get are 80 points for each skills, thats good for beating the game i belive, no need to train much higher than that.