Not sure about the cup but in the roadster i change just after the first red light as come on majority of the time, I drive her with an H shifter so the time it takes me to move my hand from wheel then engage the next gear is done before it hits the limiter. I sometimes go slightly early or late there's not much in it. The peak power is around the green light area. Using the 5 speed box in the roadster if im tailing someone in there draft Ill change before the 2nd red light is even close to coming on to help me get the pass done.
You don't need to scream the hell out of her. Sometimes you may want to run the full length of the gear on a hill for example ill change as late as possible.
If that makes sense.
Fuel the odd gallon here and there wont make a huge difference. But every bit of weight will effect, acceleration, top speed, braking times, tire wear, and handling. Run as much as you need +1 gallon to be safe no more in sprint races.
Just going to look at my Laguna time now not sure what your 1:48 time means
Edit
According to Iracing my best time in the MX5 Roadster at Laguna is a 1:43.432 with the FIXED SETUP.
If your 1:48 is right then hit me up I should be able to help you find at least a second or 4.
A guess at where you are going wrong. Braking to late to hard, taking to much speed into the bend and not enough out. Plus a host of other things that will be unbalancing the MX5 as you push her to your limit. You just have to be smooth and not loose speed, as little steering as possible. No stamping the Throttle, roll it on and off, braking very smoothly and no more than a quarter of what the brake as in it. Unless it a hairpin after a very long straight you just dab her.
Lagunas a bugger

But I do find it fun in the MX5