5100mv is the absolute max. you can keep making more power but you run out of load points. as once you top 5100mv the load axis will never increase anymore. you could in theory make upwards of 260rwkw on the stock afm but youll find once you go past near 4800rpm its probably going to flatline across the load axis, djetro style.
the rule of thumb is no nore than 250rwkw, but its not like you can't make 260rwkw with it, or 270rwkw and so on, its simply that a rule of thumb for nice tuning.
i see mine max out frequently at 190rwkw on the stock afm, but at like 5500rpm
250rwkw will certainly work, youll just have less load points to play with