Ah, okay. I misunderstood. I thought you were saying your speedo reads 110% of actual and +4 of actual at the same time throughout the entire range. I was tired last night when I read that...
Those are pretty big margins that the cops will let you get by with. Here, it's really up to the cop's disgression. He can write you a ticket for doing 1 MPH over. Or, he can let you go by at more than 20 MPH over. Just depends on who the cop is and what his mood is at that moment.
And, you're the first Boxster owner that I've read that has a speedo that's linear. All others report the same ~3-4 MPH higher reading throughout the entire speedo range. I wonder why yours is different? Stock wheel sizes?