If the tank designer is not done well, then
@Louella will be correct and the player will be stuck clicking the same choices in every game to build the exact same best tank versus something "not-the-best". Every effort would have to be made to ensure a best tank design is not possible to produce and trade offs are the rule. That is what the nations faced during the war. No one was able to build the best tank possible due to costs, reliability, resources, efficiency loss in the factories, technology, complexity, and etc...
A well designed tank designer would incorporate costs in resources, research time and timing, IC cost, and loss of efficiency when changing designs versus upgrading a current design. Some research could be, like in real life, inconvenient in timing. For example, it might well be possible to research a large 88 mm into a new turret, but the engine that can haul it around is a year behind. This is what always plagued tank and aircraft designs. The know-how to build a better tank or plane was most always ahead of the power plant that could haul or fly it. I suspect that is true for ships, too.
Having different efficiency losses for the various upgraded parts of a tank could also make it much harder to build the best tank immediately. Instead, to build the best tank would require researching multiple items/systems, each one costing a different amount of efficiency loss to incorporate on an existing production line. For example, adding better/wider tracks may cost little efficiency time, but would cost more IC, so still slowing production at least a little. Changing the turret to fit a larger gun (which was researched under guns) would cost a large loss in efficiency due to refitting the factory to forge, lift, and place new turrets. Changing the hull and turret of a tank would most likely mean losing all efficiency and starting at zero efficiency. A hard hit to take.
There is so much room to play with research right now. For one thing, there is to little of it in the most critical areas, tanks and planes, and it is to easy to build those same critical items years ahead of time. Adding more to research could change the rushing of planes and tanks into something more realistic. If anything, many players could get many hours of fun trying to build that "best" tank for those two or three critical moments in history and in numbers that matter.