Different dynos, and different correction factors. John's numbers are on a different dyno with what I believe is a full correction factor (~1.25) while mine are on a DynoDynamics with a turbo correction factor (1.13), since the full one only applies to N/A cars.
Another thing is I that I believe John's peak dyno numbers are NOT the numbers he drives around with daily (closer to 600whp), since the peak numbers were a little too aggressive on the tune. Contrarily, I was trapping 1-1.5mph slower a month ago when I dyno'd, so I'm probably closer to 380 on my same dyno - I just haven't gone back.
If you remove the correction factors and work with raw numbers like I prefer, then you end up with:
480whp = 130mph
340whp = 118mph
The ET is less comparable, since it involves tires/traction and shifting. John was on street tires and is new, so don't sweat that part. The trap speeds show the difference in power-to-weight ratio. 140whp and 12mph is about right. This just exemplifies the tenets I'm always preaching about dyno numbers. It's nothing but a number, and taken out of context, it means _nothing_. Unfortunately, the majority of people don't have a clue about the differences, and I have even been told by one forum member that he simply does not care. The downside is that some of these same people come up to me at the track and ask me why the hell I am so much faster with "lower" dyno numbers. Music to my ears...