This car is the same set-up as the older Toyota Supra, which was and amazing power plant. So the first thing that will increase your mileage with or without HHO, will be a cold air intake and a real K&N filter. I would normally say to restrict your air flow but you aren't going for that kind of system, I assure you.
There are many ways that you can go with this car, and the simplest of cells and little interference with sensors is best. Study all of you options for cells before you build or buy. On YOUR car(also Nissan Maxima's) I would not recommend trying to tap into the O2 sensors, the system is too finicky and will decrease your mileage!
Its done right or its not done !
Hail HHO.