The reason you car dumps in loads of fuel while cold on start up it because the fuel does not atomize well while cold so it adds more so it can get the vapors which is the only thing that burns in the cylinder anyways.
If you lean it out while cold you may not get enough vaporization to start the car well. If you want to start out lean then you might try heating the intake manifold with something like a hair dryer, block heater, heat gun, etc.
Most FI cars I have seen have a therm sensor as part of the fuel injection control. Once the coolant warms up the car starts leaning out. Some cars like the late 80's early 90's 626 and ford probes had problems if you got an air bubble stuck at the heat sensor. I've had that problem.
Early attempts at ethanol powered/converted vehicles sprayed the fuel onto a heated mesh to aid cold starting. This is mostly becuase alcohol is slightly less volatile then gasoline, but it can be applied to atomise any fuel better, under any condition! I wonder if diesel glow plugs could be modified slightly? or just stuck deep in the intake manifold, but not quite protruding into it?
There is one thing you have to take into consideration. When you add HHO to the A/F mix, it causes a very fast burn with no flamewall. Very little if ANY is still burning when the exhaust valve opens, so you have more efficiency from the burn.
Fact is, you could probably run regular and not experience any knock problems in an engine that would otherwise complain if it isn't drinking premium gas with a good HHO generator installed.