Zhoomsa

I figured it out. Use [spell=408303] (1) immediately, followed by [spell=408327] (2) straight away. Then fly along until [spell=408303] is almost running out. Then press [spell=408321] (3) and repeat. If you have a hard time tracking when Growth Spurt is about to run out, use the WeakAura posted by InvisiBill in this comment section. I got gold in my first try doing this.

For Zhusadormu's race you'll see these abilities in your vehicle interface:

1. [spell=408303]
2. 
   
3. [spell=408327]
4. 
   
5. [spell=408321]

I personally didn't "feel" the improved speed of [spell=408303] and for whatever reason, the slow-down caused by [spell=408321] seems to be intended. I also don't like that there's no better visual for the uptime of Growth Spurt. When it's running out before you were using [spell=408321] for the Cooldown reset, everything will be unusable and there's barely any chance to get back into the race, so you can only restart.

For my best attempts I was dumping all Vigor into [spell=408303] at the start, immediately using [spell=408327] for the Vigor recharge and more speed, and when I get close to a gale wind, reset their cooldowns with [spell=408321] (keep an eye out for your Growth Spurt uptime to not miss that, I was using a WeakAura for that) and immediately Spurt + Rewind. This way I almost skipped the slow-down.

Another frustrating whelp racing, riddled with bugs and very weird mechanics that don't do anything half of the time. It's annoying that they put an achievement behind this.