Lucky for you, I have done this exact experiment. Unlike the 1.8's, I have consistently measured our tuned 1.6 to use, on average, including going in and out of the pits, about a gallon every 10 minutes. That is at BRP 13 CW, however its all the same at most of the SoCal Tracks.
The tank holds 10 gallons, so that's 100 minutes of running, minus 5 minutes for the pit stop and you're looking at about 15 min, or about 1.5 gallons to spare using a full tank. Don't forget about safety cars. The tank also uses almost all the fuel before cutting out.
An extra 5 gallons weighs 45 lbs. That could average to 3 or 4 tenths a lap maybe? Loosing a second every two laps to the guy in front who is running a half tank.
Those tenths sure add up.
Spec Miata's run 2:06's in sprint races at Buttonwillow 13 CW, so probably 2:10's in endurance racing in the evening. So that's about 40 laps total, best case including pit stops.
You will be running an extra 5 gallons during the first stint compared to the equivalent fuel stop strategy, so that's 20 laps with the extra weight, loosing 1 second for every two laps, that's about 10 seconds disadvantage you have over the 90 minute race by not re-fueling at half way.
That's assuming you're loosing half a second a lap with the extra 45 lbs.
of course, once off track excursion is worth 25 seconds so... there's that too. haha
pick your poison!