Your chances of winning with a P9, 2/4/4/3/4 Chocobo goes down drastically as you reach R240/R300. I would say you will most likely have no chance of placing 1st in R300 brackets. Speed is arguably the most important stat for a Chocobo and since all R300 races happen so quickly, that 2-star speed will really pull you down unless something tragic happens to the other 7 competitors. R300 races have little obstacles to stop these bulldozing NPCs. Would be better to breed it at level 40.

Dash 2 and 3 are useful in lower bracket races because:
1. Quick acceleration at ~500 speed, dropping back down to your Chocobo's max speed. Allows you to break away from Briars easily.
2. 4-5 seconds of free stamina. Good to use during Frenzy/Heavy.

Point 1 becomes moot in R300. Since everyone's essentially sprinting at ~500 speed including that NPC who activated Briar, Dash won't burst you out of trouble. In the end it's a good, flexible skill to have I guess. If at all possible, try to go for Cure III. That 12% stamina refresh is absolutely amazing.