If you increase fuel pressure, you put more fuel into the engine during each pulse, which will be noticed by the O2 sensor, which then signals the computer to make smaller (shorter) pulses of fuel. Every attempt (within the authority range of the computer) you do to change the mixture will do absolutely nothing as long as you leave the O2 sensor connected (spoil sport!).

Now, if you disconnect the O2 sensor and modify the engine to digest more fuel, you will need some way to increase the amount of fuel being injected, and here more pressure is just the ticket.

Also, there is a "rising rate" type regulator that will increase the pressure ONLY when the throttle is being opened. This can be useful since normally our engines run too lean under acceleration.

Of course, you could just disconnect your O2 sensor (letting the computer use the programmed default value), calibrate your AFM for aged components, and get probably 98% of the benefits of a rising rate regulator.
Bob

Does anyone have any experience with one?
Is it worth the $200.00?
I have an '85 535i with 116,000 miles - stock with a cold air intake. I don't plan on doing any major engine mods.
Thanks.