Yep, you're reading the timing curve right. I was thinking the same thing you were, so I had asked them to run some 108 octane and put the timing curve back in that the car that it was delivered with (33° all in by 3000). That timing curve only made 149ft/lb.
This is the only L-series I've seen that needs only 26° to make peak torque. Seems a bit odd...
I'll double check the cam and make sure it's installed straight up as per the builders recommendation. I didn't actually check this before sending it off to the dyno as it was set by the builder. Maybe I shouldn't have assumed, but I'll check it.
I'm with you guys on the 'not enough air' theory based on how the torque falls off a cliff at 5000. Having said that, I think the carbs should be good for a bit better than 156hp, but given my lack of experience with these engines, what I 'think' doesn't exactly carry a lot of weight. I'll go do some more searching and see how people have been doing with the N47 head as well.
Thanks for your comments guys.