RFPR increase pressure with and increase in pressure, this is correct. But remember your manifold pressure is below atmospheric at idle (vacuum), and it INCREASES as power increases (ie, the throttle plate opens, and pressure increases to higher absolute pressures.... ie lower vacuum). So RFPR do work for NA engines, you just have to source them correctly (I've used one on a 600 cc na cycle engine).
Also, I've read several papers that state that maximum power is obtain at slightly rich (or lean) conditions, not stoichiometric. Can't remember if it's rich or lean, though (most people will say lean.... that's what I'd guess, too).