As far as I know these changes work in Single Player only (Skirmish and Campaign.) I would assume they will not work in multi-player.
There are other properties listed in the proto.xml file, such as resource cost for each unit, etc. Presumably changing these other values could also alter build time for units as well as resource cost.
This is far from a trainer, but it's the best method I've come up with and so far it's working great. Editing these properties and combining it with the cheat codes for the game, might just suffice.
If you have any problems, questions, suggestions, or information regarding editing files post here or send me a private message. I'll respond as promptly as I can.