Adam
what you really need to look at is pressure and water flow. just like jet washers its not the psi but the amount of water that you put out. this is why the match between water flow and vacs is so important as get it wrong and you over wet a carpet. I have only had one a machine that had a water flow valve on it and that was my ninja. (which I would recommend)
I think many manafactors stay away from this because in in untrained hands damage can happen. by buying a machine thats had the numbers done on the flow, psi and vacs. makes things a lot safer. so by buying machine hose and wand from same place. I seen machines with wrong type of hose, it all matters
Now lets talk about emulsification if you are using the right type of surfactant then the dirt will be emulsified and will just need the right amount of water to flush it out. this will also be affected by the level of soil needed to be flushed out. over wetting happens when emulsification has not happened. by keep going over and over the same area.
so given a light soil carpet with the right detergent not much water. heavy soil will take more.
I would advise a prochem course before buying. they have the best course that teaches about matching method, conditions, and products.
Good Luck