OK, cool. I just wanted to make sure I understood your concern. You won't have a problem with your valves overheating. I recommend you not run the EFIE that high, or at all, should you turn off your electrolyser for any reason. But you are fine with your current set up.
I can't answer your question about adding more HHO. But I can tell you something about that. HHO, when burned, does not give back as much energy as the energy that was used to create it. There are several conversions involved, and energy is lost at each conversion. These are: mechanical to electrical (alternator), electrical to chemical (electrolyser), and chemical to mechanical (burning the HHO). If that were all there were to the equation, HHO wouldn't work to save on fuel.
Where we get our gains is the fact that the HHO catalyzes the gas and causes IT to burn more efficiently. We pour the majority of our gas out the exhaust pipe, and the HHO causes some of the waste to be used in the combustion process. This gain in efficiency of the petrol is where we get our mileage gains. However, only so much HHO will give us more efficiency in this way. Adding more after that, you'll start to lose mileage because of the efficiency losses. I wish I knew how to tell you the exact number of liters/min is the correct amount for your car, but I can't. The only way I know of to find the exact amount that is correct is by trial and error. As you add more gas, at some point you'll hit a peak, and then your mileage will start to decline. The correct amount is the amount you were using at your peak mileage.
That's probably a longer answer than you were looking for. But that's how it works.
As to your MAP sensor: Yes you can do adjustments to it and it will help your oxygen sensor. In actual fact, it can happen that your computer will get wise to your adjustments to the oxygen sensor unless you do make some adjustment to the MAP sensor too. This varies between manufacturers. However, it certainly wouldn't hurt, and is bound to help. Please see Tuning For Mileage
for more information on this subject.