I also tried much higher values, and here's my experience so far:
If you make materials anymore than 20, the AI won't build pioneers at all, this is apparently due to how the AI decides what units to build, it is based on material and metal cost, and the most "expensive" units will only cost 20 material/metal. This is actually an okay setup if you like to play on tiny maps, as it will end up being a battle of capital cities most of the time.
At 20 materials, some AIs who start near materials has a big advantage. They will still expand like mad while others will be stuck with 1 city. This is also true to some extent with 10 materials, although it is admittedly less of a problem.
If you add metal to the cost, some of the AI will be screwed (that doesn't have an iron mine at the start) and end up not building pioneers at all. They would need to get lucky by picking up metal in their exploration, or start with resourcefulness.
Adding higher pop values generally make factions with royal sovereign too strong, since other factions will gimp themselves by making pioneers.
Gold and training time seems to be the only "fair" values thus far. I've tried 200 gold with +10 turns, and 300 gold with +15 turns, both were pretty successful at stopping massive city spam. However, the problem remains in how the AI selects where to build their cities. That is to say, instead of reaching out further to grab more land, they will still build their 2nd city near the closest unclaimed resource despite the increased difficulty of getting a pioneer.