1st thing I did when I got the game was gold all the licenses... took 23 hours of real time, around 12 hours of actual driving... GT5 license tests are simple compared to previous versions of the game and only 1 or 2 are a decent challenge.
Licenses are still a great way to learn the basics of the physics engine and how to get a good line/speed through a given corner. Personally, I think everyone should do them - they'd become much better drivers if they took the time to learn how to play the game properly.
And all SRF does is up the grip levels, so this just means you complete the test a bit faster than your would with it off... you still need to use proper driving techniques to pass them (well, to gold standard you do).
After the licenses I completed all the special events in order, only running A Spec when I needed to to level up and un-lock the next special event.
By the time I got to the main part of A spec I was already at a decent level and had a good stock of cars to get me going.