That zombie gun you're looking at is really a re-labled Jing Gong. If you buy from either of the first two places you linked they will be able to change the spring for you, just tell them what FPS you want it to shoot. As you mentioned 350 fps is the common limit for CQB, 400fps the common limit for outdoor. I've heard many warnings to stay away from the third site you've linked to though I have no personal experience with them myself.
JG's and other chinese clone guns can be made into nice shooting guns with a relatively small investment into internal parts (and some knowledge of mechboxes)but their externals will always be inferior. A better quality brand will come with much nicer and more durable externals which are more difficult and expensive to replace, especially in this country. When I buy guns I will generally try to choose the ones that have the nicest externals and are free of proprietary parts.
Last edited by Kozzie; February 20th, 2012 at 14:04..