The reason why the power is low is because the run was terminated early. I'd imagine there was at least 10% more power to be had had he rev'd it out to 7k.
This would give you 160bhp ish.
DD roller read fairly accurate/low.
The conditions also weren't perfect judging from the correction figure, which can throw things out slightly.
As people have said though, you need to look about getting some for of management, this will get you a lot more power/safety and fuel economy.
