I think MFive is only asking for stat caps to be included in the rulesets, although that's an interesting idea Volutar.
Lets clarify our terminology. The smallest amount of TUs that soldier can have when you buy them is the TU spawn min and the most they can have is the TU spawn max, because when you buy a soldier she "spawns" with some amount of TUs in that range. When she kicks ass on lots of missions and gets her TUs up as high as she ever can, the maximum amount of TUs she can ever achieve (without cheating/hacking) is the TU cap.
Currently in the ruleset each stat has a min and max spawn value that is used for all soldiers. In MFives idea (and common sense, it's probably in the works) each stat will have a min spawn, max spawn, and a cap specified in the ruleset. Basically, the caps are missing from the rulesets because SupSuper hasn't moved them yet.
In Volutars idea, which will likely have to wait for 1.0, the ruleset would have min-max spawn and min-max caps for each stat and each soldier would have their own caps, rolled at purchase, which determine the caps used for that soldier.
In my idea she would probably roll 45 TUs from the spawn min-max and 58 TUs from the cap min-max, because she has a vagina and I'm sexist.