I run a 12:1 FMU+inline right now, and I constantly hit 2-3 lbs boost in closed loop.

Although the FMU increases fuel pressure, can the stock ECU in closed loop
function fast enough to keep stoich when I boost on part throttle?

Also, why do we need open loop mode? What parameters are used in open loop?

Does the gas pedal actually increase the amount of gas injected proportional to throttle %, or does it only open the throttle body.. which in turn changes the a/f ratio.. the ECU sees the change and adjusts fuel accordingly?

 

This article should help you to understand what/how a speed-density type system works.

The TPS is used to tell the ECU 3 things [AFAIK]:
1) When the throttle is closed: idling and fuel cut on deceleration.
2) How fast it is opened....or the rate of change. This is used by the ECU to inject a little more fuel when the throttle is snapped open.
3) When the throttle is Wide Open.

Open & Closed Loop refer to how the ECU is operating. When in closed loop, the ECU is paying attention to the O2 sensor to constantly adjust the mix for emmisions and clean running. When in open loop [during cold start-up and WOT], it ignores the O2 and reverts to internal values for fueling.