I currently have around 2cm above my head inside my truck and find that it is plenty. The only disadvantage is that many guests have to stoop. The reduced height is a huge advantage regarding trees on narrow forest tracks, bridges in old cities, and low power lines in less developed countries.
I am 181cm heigh and so the box in my truck is likely to be usable for many others if I ever choose to sell it. It would probably be unwise to invest too much money in a box with headroom of much lower as you will be reducing your potential market if you ever come to sell the truck.
My truck is around 3.5m heigh, I would say that I could have added another 20 cm without major complications, but any higher would have caused problems regularly.
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