If that's true, than why do people get higher psi with accurate vaccuum/boost gauges?

You know why? Because the OE wastegate with the actuator arm at it's stock position allows some boost to leak while at full throttle.

Yes boost is controlled by the ECU, but not exclusively.

"Otherwise why would one need to modify the ECU in the first place?"

One needs to modify the ECU because simply not allowing the wastegate to leak as much or at all under full throttle doesn't also change the other parameters of the engine that. After a certain increase, unless the parameters are changed, not enough fuel will be injected, ignition timing will be off from optimal, as well as proper valve timing, thus leading to a lean fuel to air ratio, which can cause damage. That's why many don't recommend over 3 turns to begin with, then if the engine is running fine still, try another half turn, then another, etc, until one pull's codes. When that happens back the actuator arm off.

Have you actually seen numbers on 850's with actuator arm adjustments? I've seen an S70 T5A SE and two 854Ts with actuator arm adustments. The S70 had a Haltech, two had GReddy's. The S70, and one of the 850's also had GReddy Air/Fuel ratio gauges. The both picked up psi throughout the rpm range, and both were still within operating parameters in regards to air/fuel. The S70 in question is the same S70 I mentioned with 18" x 8" Borbet rims. He has since welded his wastegate shut, and installed a blow off valve to hold 18psi of peak boost while also able to blow off enough boost between shifts on his auto trannied S70.

At the time of the waste gate adjustment, he only had a 2.5" cat-back, open element air filter, 1 coil of suspension clamped, and gauges. Now, he has a GIAC chip, 3" complete exhaust (downpipe to tip, with a 5" round chromed with rolled edged tip on a rear mount muffler), welded wastegate with a TIAL blow off valve, a number of turns on his wastegate, the iPd black mesh grill, blue bulbed head lights, DARK tinted windows, V70 R AWD front fascia, stock tranny, no other suspension or brake modifications and a shaved rear end except for a T5-R sign. He hasn't had it dynoed yet, hasn't raced it yet, hasn't had it clocked yet. All we know is even with the increased wheel diameter, his Yoko Paradas are taking major abuse.

And the highest boost he's seen is about 18psi.

Don't believe me, head to Magnus Motorsports on Keele St. just south of Rutherford Rd. and ask for Marco (the mod guy) or Rob (the owner of the S70) and ask them. They've been working on his car non-stop for the last little while.