Its taken me a while to learn it, I think I'm finally getting there. I had the same feeling, I felt like my runs were clean and fast, yet I was in the 3'30's and the next guys were a few seconds faster, with the fastest on my list @ 3'25.xxx. I have gotten my times down, and now in the BMW I'm at 3'21.xxx. I think the Porsche is faster, or at least more easy to maintain a higher speed.
Much like a racing sim, make the corners as straight as possible, out in out. Only drift on the corners you have to and don't turn any more than you have to. Every touch on the wheel will drop your speed. Drive into oncoming traffic/draft/near miss and only boost until your speedo goes red, and just use the boost to maintain the top speed as need be.
Only use the shortcuts that are faster. The shortcuts take a good chunk out of your top speed. There really is only one shortcut to use for the fastest route. When you come up to the long winding bridge near the end, you don't take the first left off into the shortcut, you continue down the straight, drift the left hander then take the shortcut to the right down onto the sand. Follow this shortcut all the way around until your forced back onto the road for the final stretch.
Don't hit the walls, don't hit anything, as it slows you down too much. After a corner, boost back up to max speed, then just maintain it while building up more boost by driving into oncoming traffic so when you have to drift again, you have plenty of boost to get back up to speed quickly.
Needless to say, I'm loving the game and definitely a day one purchase.