I would expect [0, 0] to be the upper-left, not the lower-left.
In any case, using the new projection matrix, [400, 300] is exactly the same as [0, 0] in the previous projection matrix (center of the screen).
I would suggest using that as your test case as it may very well be that your particles are all shooting upwards and since your Y is 0, all particles start at the top of the screen and fly even further off the top of the screen, meaning you can never see them.
L. Spiro
I don't think so, I changed to Z axis and tried 0,0,0 for the position and it's still at center of screen... If I use 10,0,0 it move way over to the right side barely visible...
So for some reason the uploaded projection matrix isn't working?