First of all, dyno numbers are a tuning tool and nothing much more beyond that. It is widely accepted that dynos will vary in their readings by as much as 15% from one to the next. 15% of 427 rwhp = 64 rwhp.
Second, I've got 8.5:1 compression and only 7.5 psi boost.
Those three things more than account for our numbers being what they are.
Third, tuning (unless you want to blow it up!) will only change your hp output by maybe +/- 5% depending on what timing and afr you decide to go with. That is going to be determined by how conservative you want to be, what your setup is, and what octane fuel you use.
A tuners real skill will show when you look at engine life and driveability.
People who measure up their tuners by how much power is shown on the print out are true amateurs.
If a tuner wants to show you more hp on the dyno, there are plenty of ways to do it without changing a single thing in the car's tune. The dyno has a lot of variables that can be screwed with to show bigger numbers.