If you have very low compression and no chance of detonation or vapor lock then sure, leave the heat crossover, it will vaporize the fuel better. On the other hand if you are trying to get maximum efficency through very high compression then block it or it will increase the chance of detonation.

Water injection does nothing but slow the combustion process like EGR does, unless you are running into detonation don't waste your time, it will hurt MPG. Al Gore would be happy if you do as it reduces NOx emissions by reducing combustion temps. It will like more spark lead (negative work done against the rising piston) and it will also mean less molecules burned before the exhaust valve opens and further combustion of those molecules already in there will not produce any power, you want to burn as many of the fuel molecules in the chamber as possible before the ex valve opens for peak efficiency.