Well, of course the first problem is the creation of NOx(which is helped by water injection). That aside, as far as I know, higher advance makes for more complete burn of the fuel available. If you start burning the fuel earlier, more of it will combust to follow the piston all the way down. The biggest problem from what I have seen and learned on here, is if you advance the timing too far, you will end up with too much resistance against the starter motor, (because the injectors are firing at the pistons while they are on their way up), creating high current draw from the battery, overheating the starter, and not enough cranking RPMs to get the engine fired. A high perf starter, or dual batteries may help this but the better way is to alter the advance in the IP. There are several mentions here about it, and there is a long discussion about it on the other forum, it has a lot to do with internal pump pressure and how the advance mechanism works in the Injection pump. Overall, you want to start with a near stock initial cranking and idle advance, but then you want the overall advance to be higher than stock.
my 2 cents
-T