I actually expect Siam to become a powerhouse even in vanilla with the next patch. They get +50% culture from city states which is extra-useful without being able to stockpile social policies. Why? Because you will stockpile money instead and bribe all available cultural CS once you hit the Renaissance. Your Wats also produce extra culture because they come online at about the same time. Having them yield 50% more culture is strong. Why 2? Because Siam gets +50% food, which makes it more likely to get you over the rounding threshold, like it already is in this mod. I'm not advocating nerfing Siam, by the way. I merely want to point out that it is maybe the most underrated civ because they not only have a good ability but the rest of the package is also quite good. In this way they are similar to China, who also have no extremely good special ability, unit or building but since all of them are good the civ becomes very strong.