Alistair,
I think that Mr. Marshall's HP figures are close enough for government
work.  Manufacturer's posted HP & Torque figures are not like an
aftermarket bolt-on, gee-whiz kinda outfit's figures.  I am sure that you
would agree that in today's litigation-happy society that a manufacturer is
quite unlikely to publish incorrect figures.  Mr. Marshall's figures are
right in line with the bulk of the data that I have accumulated (note - not
tested myself) over the years.  The bolt-on/hop-up community that claims
10% more HP or 20% more Torque, I am more than ready to doubt but I would
believe a manufacturer's report is close enough to allow fair comparison
between manufacturers.
Mike