I have a mechanical boost gauge on my truck. I run 91 octane always in my truck and the only mods are AFE intercooler pipes and Turbosmart BOV. Stock Tune.
As far as max boost, it depends on the temp outside. The most I have seen is in warmer weather where I normally see 21 lbs. In the winter, it will max out at somewhere between 18-20 lbs.
I think that the main thing that a tune changes is how the boost ramps up during RPM. On my stock tune, it seems like it jumps up to 15 lbs. and sits there until high RPM. You can see this in the dyno graph for the Ford Tune where it did not pickup a ton of power overall but there was a good increase at lower RPM.