Are there any options for changing the final drive ratio in the S54 transmission aside from using the 3.944 ratio from the S51? I'd probably be looking for a ratio somewhere between 3:1 and 3.5:1 (land speed racing).
The final drive is made up of a gear that is part of the output shaft on the tranny, and a ring gear. The only way to change the final drive is to replace the entire output shaft. So, it's not nearly as easy to swap the final drive in a FWD or MR2 tranny as it is in a RWD car with a separate differential.
Look to possibly using the S51 gearset with the Celica final drive if you need more length, though it looks like that might be overkill. We are of course assuming you've got the power to push the car this fast. At 7k rpms and 24" OD tires in 5th gear, the stock MR2 S54 will hit ~146mph. The Celica will get you to ~163mph. The S51 (Camry) will hit ~173. The Camry gears with the Celica final drive will hit ~183. For comparison, the turbo tranny will hit ~160, the Solara/Camry V6 tranny ~173. The turbo/V6 tranny and axles is gonna weigh probably 75# more.
I'd probably go with the Celica setup. I'd imagine you want your sweet spot in the power curve to happen right before your target mph, rather than during or after. You can always change tire OD to fine tune.
I understand the 5th gear in the S54 is 0.83. What's the Camry's 5th gear ratio? With the 0.83 5th, the 3.737 Celica final drive, and 195/55R15 tires (or equivalent diameter) the car would do 170mph - the target speed - at 7500RPM. Necessary power would be about 240hp at the engine if the Cd can be lowered to 0.29 with mirror removal and serious lowering (no body mods allowed in the class). I'm quite sure the 3S-GE is capable of that Weight is not very important incidentally, on the short course you have two miles to get up to speed - mile three is timed for your official speed.
I believe the camry is a 0.731, super high. Too high for what you're trying to acheive. I'd keep the gears close and raise the final drive like you're suggesting, then you'll lose fewer revs during shifts.