Just a thought; Are all variant stats simply linearly more effective the higher up you go, or will there be combinations or threshold values to consider? If for example you'd need a new fighter variant to tackle strategic bombing, you'd probably need a minimum engine strength to at all be able to reach their altitude. So variants with 0, 1 or 2 in engine upgrades might be around equally ineffective in intercepting bombers because they're beneath some threshold value in actually reaching necessary altitudes. Another example is making bad combinations of upgrades perhaps by up-gunning or up-motoring a variant without increasing reliability, resulting in a less effective overall variant from constant engine and weapon trouble.
It seems like it would be a bit more interesting to use if something like this was the case, and less obvious which was the ultimate configuration.
It seems like it would be a bit more interesting to use if something like this was the case, and less obvious which was the ultimate configuration.