[TokyoSeven]

Interesting questions. It is quiet possible your mechbox would survive that FPS

Do you know the current FPS of your gun? Jing Gong guns are more often than not already somewhere in the 350-400FPS area. Its quiet possible to install an m120 to achieve the FPS you wish, if you decide to do so please also take into consideration that it would be wise to install a quality bearing spring guide and some good quality metal bushings if not already present and a reshimming,thats just the absolute bare minimum. The is definately more you can do. However Im not going to muck about explaining what parts to get and what not. I would like to jump back to the possibility that your gun may already possibly be near the 400 FPS area.

Lets say for example your current spring yields 370 FPS. Why not just simple install a new quality hop up system and a tightbore barrel. That alone will get your closer to your goal if not achieving it all together. Its possible to swap out other internals to achieve your goal FPS as well. Replacing anything related to the compression system of the mechbox with an upgraded quality part will boost your FPS as well. With the installation of new quality durable parts you should have no worries about the longevity of your mechbox. Do your best to try and keep all the parts by one company if you can, its usually best to do so since a company usually would make its parts to work efficiently together. Mix and matching multiple companies parts can work however you then increase the possibility of failure due to increased differences in companies tolerances when it comes to sizes. As long as all parts are installed properly you should have no issues.

Edit:

If I may I would like to recommened two other companies for springs if you are open to suggestions.
Either prometheus or modify, both are excellent quality companies and both companies produce excellent consistent springs that will stay consistent over time.