[Joran Dax]

The care plan you outline would be a "Hybrid"--mixing a mental with a physical. You sacrifice some work in the process. The most efficient way to do this would be to mix Messages with Digging once they become available.

EDIT:
I can't remember whether Messages lowers DIS or REC... and I can't see the careplans on FFXIClopedia right now (can't connect), so you might have to use Packages/Digging instead which wastes points.

You see, this is the part I just don't quite understand.

1) Abilities learned through high Discernment levels can affect how well you perform in the races
2) STR and END also affect how well you do in the races

Now the problem here is that pre adult phase, enhancing one or both physical attributes will decrease both mental attributes by an equal amount of physical gain, and vise versa. Its not until you reach the adult phase that you recieve care plans that let you dial it in specifically, which means for half your life you have very little choice in the matter, its either one or the other.

Granted the Delivering Message option lets you increase strength while only losing receptivity which would seem to have little or no bearing on racing, but using Digging for Treasure at this point to increase your discernment is a detriment to your chocobos endurance.

There doesn't seem to be any way at all for you to keep discernment or endurance without sacrificing the other. Which leads me to believe that both endurance and the discernment abilities will have a similar or equal effect on your chocobos racing abilities and all things being equal, the birds strength will be the major deciding factor. Its either that or endurance will have no bearing at all on the chocobos racing ability.