The regulator on a metro is built into the throttle body I think. So really the best bet is to modify the existing one. It still has to work and adjust the pressure based on manifold vacuum or it will really mess up your tune.
Really though the car is running good and getting about expected mileage out of E10 so I doubt it will do much. The only thing it would give you is a bit more room for the car to have something go wrong before it kills your mileage. Still not worth spending money on blindly but if you can do it for free...
As far as the increased spark. My metro has an msd 6al and a really high performance coil on it and it doesn't change the way it runs on ethanol at all compared to E0. I am not really sure what is needed to actually get better mileage on ethanol. I figure it will realistically have to be a car that was designed to run on it to actually get the only 3% loss that is expected. All the older stuff will just have to deal with it or remove it from the gas.
I am still planning on building a setup to remove the ethanol from the gas and hopefully be able to do about 25-30 gallons at a time. I just have to figure out the ideal way of doing it.
So how about a large glass fish tank that is sitting with one side like 5 inches higher than the other and a hole in the bottom on the low side going to a clear hose with a ball valve on it. That way I could put gas and water in there and once they are cleanly separated I could slowly open the valve and drain the water away till it is all gone or just in the tube. Then pump the gas out into my tank. Fish tanks are expensive to buy new but I could probably make something good enough out of glass and just epoxy it all together.
Here is what I am basing my idea off of:
500 ml Separatory Funnel Separatory Funnel : United Nuclear , Scientific Equipment & Supplies Just much bigger.