I think even with the proposed change (100 faith, 25 gg/ga points per 2 pop) it will still be considerably worse than most other founders while still requiring more effort to gain yields than other founders. Like others have said, when you're warmongering, you're not usually lacking GAs/GGs (not to mention that as you warmonger and get more and more GGs/GAs, you need more and more GG/GA points to generate the next one, so the bonus GG/GA points from this founder belief will result in very few extra GG/GA along the course of a game) and value of extra faith quickly pales in comparison to other yields, especially since you probably won't be using faith for missionaries to spread (because you'll be spreading with inquisitors in your newly conquered cities).
Also, it won't be 3-4, 5-6, 7-8 with equal gains, but 2-3, 4-5, 6-7, 8-9 etc.
Furthermore, let's say you conquer a city with 18 pop. That's quite high for many AI cities (except for very late eras), which means it won't be that easy to conquer. According to the proposed formula you'll gain 900 faith and 225 GG/GA points. At that point in the game, that will probably be enough faith for 2-3 missionaries and a not-that-big portion of a new GG/GA (if it'll be your 5th GG, it'll be around a quarter of a new GG, if I remember correctly). That hardly seems like a worthy reward for conquering a relatively big AI city, with the majority of yields not coming in the early eras when they matter the most.
So I personally don't need to test out this formula in a new version, because the numbers show it will be worse than most founder beliefs, especially considering that some will receive another buff (like Holy Law).
And lastly, I think it's odd that a Founder belief (that only Founders benefit from) geared towards warring style is significantly worse than a Reformation belief geared towards warring style, when that belief benefits other civs as will.