While pondering about various japanese aeronautical what-ifs, i reminded myself about their late war single-seat twin engined fighters (such as Ki-83, J5N etc.) as opposed to other (and more widely used) two or even three-seat ones (like J1N, Ki-45, Ki-102 etc).
As far as i can make out, the only really successful twin-engined fighter that could hold it's own against the single-engined ones is the P-38. So i am thinking, and wondering what you think, did the weight of the gunner and second seat were of any significant benefit for designs such as Ki-45, and further afield, say Me-110 and designs such as the french Potez-63C2? What if Me-110, even Me-210 and so on from Germany, the Ki-45, J1N from Japan and so on were as a base design single seat? Would the performance increase be worth it and make them better able to face or evade single-seat fighters, and make them in general more successful and more effective machines? How much of an increase in performance we can expect?
I realize and recognize that specialized versions like night fighters needed to be two-seat in those days, so in this TL, the two-seat version could be a separate variant.
Thanks for your input.