the speed at which the power curve drops off is a major factor in when to hold a gear to the rev limiter
Yup, and that's exactly the calculation that Sidekick performs for you in real time
I downloaded and now regularly use Sidekick after you recommended it during of the MX-5 Cup races. Am I right in saying that when the shift light turns yellow you're in the peak power band and you should ideally shift before it turns red? I've left mine at the default settings but I've been more of less shifting when the light turns yellow so perhaps I'm shifting too soon as well...
Well, I think you probably are indeed shifting too soon, though only by a very small amount. I can't remember how low the default values are for yellow and red lights when optimal shifting is both switched on and available, maybe 96ish for red? But yeah, the best way to have the lights set up depends on preference and also somewhat on the car - this is because if a car has a power peak just before or at the redline, you can easily hit the limiter before you notice the red shift light. In a car like the MX-5, holding a shift a wee bit too long is not such a concern, as the redline is miles above the shift point in 4th and 5th (likely 3rd too).
If yellow is set to come on at 94% by default (like I say, I can't recall) and you shift the instant it comes on, then you'll be shifting into a gear where you'll have 6% less power than you had in the current gear, and thus 6% less acceleration, but since that shift is probably only a second or two early, the impact is pretty limited... (I posted acceleration curves on either this forum or the AC one a few months back for the MX-5 Cup showing the impact of optimal vs. redline shifts.)
Personally, I think I tweaked the red light to something like 99%. This means it'll come on when the power in the next gear up reaches 99% of the power in the current gear. You might argue that it's OK to use 100% in a car where you KNOW that the important shift points are way below the limiter. Btw, not sure why the defaults are set so low, so it's always possible that I've missed something
Maybe
@Topuz thinks that people are really slow at shifting and wants them to have plenty of time to notice the light