I also use snaking and the bridge mod which go excellent with this.
There is no way that I know of to adjust the default growth given from food surplus, it may be hard coded. However, these are some of the things I do, even if I do not have the sovereigns call spell which gives +20 to growth, it is imperative to get a commander hero. Your sovereign gives +10 to growth, if you didn't know, also the tower of dominion gives a bonus to growth so your first town will grow quickly if you have your sovereign and a tower of dominion in it. Also, a commander will give a bonus to growth as well, so growth actually is quite reasonable, even without sovereign's call. Also, later you will get the "Growth" production option which will also increase growth. It is intended to work this way because this makes growth a serious choice that you need to make. Do you keep your heroes at home? Do you use sovereigns call instead of another spell? Do you use the growth production rather than build or train or something else? Can you afford to train another soldier unit given your population? These are all real choices that need to be made and are meaningful.
Also, there are various buildings which give bonuses to growth if you weren't aware. Wells, inns, theatres, arenas, all wonders, etc. give bonuses to growth. So rushing to get some of those wonders is also a consideration to make.
As far as champion leveling, at first I also thought the xp gain might be too much but then I realized something. By the time you have cities that give a lot of XP, your first 2 heroes are going to be already on a high level so it wont matter that much for them. New heroes though will level quickly to catch up, making them useful. At first it will seem like it is going quickly but then you realize it isn't that big of a deal. Also, given the fact that there are so many abilities and such to choose from and for heroes to be even more meaningful they need those high levels. Keep in mind that the AI is also leveling quicker and so are enemy units as are yours. Also, It makes sense that by mid game, you should be on mid levels (20-30) while at the end of the game you should be higher and have maxed out your heroes. In all my games, it has pretty much played out like this. If I lower the city XP gain, the heroes that suffer are those new lower level heroes you get, it wont affect so much your sovereign and first hero because they will need a lot of XP to level and you will probably be using them in battle a lot anyway. Also consider the fact that when a hero is sitting in a city, each turn is a week. So after several weeks of training, shouldn't they level up? Or after a hard fought battle or two, shouldn't they level up as well?
Try it out more and let me know what you think after reading what I said. If you still think the way you think now, and if more people want me to change it, I certainly can do so. It won't be difficult.
I can definitely add in another tax rate. In the next version I will do that. Also, I always play on a modified hard difficulty (Huge bonuses to economy but no health bonuses and also higher intelligence), huge map, dense monsters and dense shards, challenging world and 12 AI players.