The marui japanese line is specifically designed to run at 300fps. No more. So they are not built to handle the stress of a 380fps spring.
There are ways to get around it, notably the ARS sorbo cylinder head which allows you to run a marui V2 at 400fps. But other things may break like gears & bushings
FPS mean very little when it comes to range, so just use .25g BBs and install a guarder clear hop rubber.
Friend of mine has a stock marui P90, all I did was put a guarder clear rubber in it and it shot 160ft easily and fairly accurately