A 400 can be "tuned" to spot on 78bhp and 145kg half wet easily, and if they did then get "whupped" at that they'd change the regs again, a capacity limit of around 70% of a 4 stroke is plain common sense, the fact there are no modern 2 strokes over 250cc is the only reason they haven't set a seperate max capacity for them.
Or they could just rejig the "engines cases from the same model" to read "cylinder block(s) from the same model".
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