Nitpick,+50% to settler production comes to 2/3 price, not half price. Anyway, you're probably right that Brazil is the most reliable. But I'd still think that Spain or Shoshone or maybe Maya have the higher possible ceiling. Brazil might have the best average time, but the best theoretically perfect Spain/etc game outraces a perfect Brazil game.
The problem with upgrades for camp clears is the time spent in doing so. It takes a good 4-5 turns for even a comp bow to bombard a fortified unit all the way down. Double that if the camp spawns another unit while you're doing so. More if you have to heal too after. That much time lost translates into more ruins missed. And 25 gold isn't much at all. A good enough CS snowball would be worth it, yes, but that's pretty chancy in getting set up right and in having another quest to get up over 60.
The problem with upgrades for camp clears is the time spent in doing so. It takes a good 4-5 turns for even a comp bow to bombard a fortified unit all the way down. Double that if the camp spawns another unit while you're doing so. More if you have to heal too after. That much time lost translates into more ruins missed. And 25 gold isn't much at all. A good enough CS snowball would be worth it, yes, but that's pretty chancy in getting set up right and in having another quest to get up over 60.