"If I got new turbos would I expect to see an increase in performance (everything else being the same)? I think not. All that matters is the boost level, right?"
No. Differently designed turbos have different thermal efficiencies for different mass flow rates. Two different turbos could have vastly different outlet temperatures for the same pressures, which effects performance considerably. You can crank up the boost high on stock turbos too, but the whole reason that it's pointless is that the temperatures even at a pretty low pressure start increasing at a rate such that the temperature increase starts having a stronger negative effect than the benefit of the higher pressure. This is due to the fact that both pressure and temperature have an effect on the density, which governs how much mass of air you are pumping into the cylinder, and thereby the amount of power produced.
