Hi,
I am also building an Optima runner, but I will use some of my remaining brushed stuff. Thinking about 13x1, that should be close to your 17.5.
I find that brushless motors are rather tolerant with ratios. You can throw in whatever pinion you have, and if the car is not quick enough, just gear up, it will not affect acceleration. Run a few laps, then check motor temps. Too low or too high ratios makes the motor hot, so that should be closely monitored to find the best ratio.
Don't think too much about what we used to run back in the days. Motor magnets were weaker, brushes were soft and low quality, and I won't comment about batts!
Optimas are soo cool! Please post about your runs.
