I just deleted the reply I was going to post. I'm going to be more brief this time.

I would caution you against making blanket statements about efficiency vs boost pressure, because that says nothing about what the flow is. That turbo MAY be efficient above 20 PSI on some engines and not on many others, because off the differences in flow.. I guess the part that got me was "on most 4cyl", because then I know you aren't talking about a specific engine (or engine size, etc). Put that Holset on a V8, and it's going to be at a completely different efficiency at 20PSI than on a D16Z6 or something like that.

If I told you Integras make the most power at 63 MPH, that would be pretty much the same thing. Not enough information, and you can't make a connection between speed and max power without knowing that information.