I got a 17" MBP also, and I met the requirements or so I thought.
Once I set the thing to 5 GHz, was I ONLY able to see a connection max of 270 Mbps.
I don't use a DIR-655, but rather a DGL-4500.
When in 2.4 GHz, and it reporting using the 40 Mhz channel width, I only see a max of 130 also.
I'm not sure why that is, but yeah...it's capable of it...just doesn't do it.