Wow! Dude, after about 30 or so sylkis greens, your chocobos are maxed. There's no sense in feeding them anymore of those expensive greens (initially expensive, anyway).
Yeah, just getting them to A level is good enough.
The long course, I guess, is supposed to be advantageous for chocobos who are, perhaps, not fast but have a lot of endurance. Personally, I always play the short course. When you're trying to race your gold chocobo up to S class, it gets really boring waiting for that second lower class chocobo to reach the finish line.