How Fast is Too Fast?

No such thing as "too fast." 

What is your speed (or wattage) at 90 RPMS vs 100? You should ride at the self selected cadence that is comfy but also gets you from Point A to B the fastest. I like to ride at 100 - but I am fastest at around 86-90. So, you can guess where I ride in a race. :-)

You are doing the correct cadence into the wind: whatever is most comfortable.

And yes, you are selecting the right cadence in the races as well: whatever allows you to run your best. Of course you don't want to spin at 100 rpms, avg 15 mph and run 10:00 miles vs. spinning at 90 rpms, avg 18 mph and running 10:30/mile. You need to find that balance. The second option is going to get you to the finish line quicker even if your run does suffer.

You are very comfortable at 100 rpms, it seems. This is a good reason why you should train all cadence ranges - from 60 rpms to 120+ rpms. We don't need to be great at all cadences, but you need to be able to use all your gears and all types of cadence ranges. Mix it up a few times a week - that's how you improve across the board....

it's all muscle memory and basic physics.

power = (torque X RPM)/constant

the constant is 5250 if you are using ftlbs and Horsepower.

This points out a few things.  First off, you can get the same amount of power out of low cadence high torque or high cadence and low torque.  The former would be what an anerobic sprinter could put out while the latter is what an endurance rider would want.  Also, your torque output is not linearly correlated to RPM.  Meaning if you increase your RPM by 1, you could either increase your torque, decrease it, or have it stay the same.  Your legs have a torque profile that is different for every rider.  Since they don't change at the same rate, this also means that once you pass your peak torque RPM, even though you are making less torque, since you are riding at higher RPM, you could in fact still be making more power.

Throw in gearing and it gets even more complicated, as you hit the point that spinning high cadence at a point that you are pushing less torque may in fact be producing more power then if you were to shift and drop your cadence lower.