Hello Matthew ........that engine vac is LOW , been working on a 1200 corsa today it pulled 21 1/2 inches of mercury no problem .......a v8 should pull that easy .
Now... engine vac loss can be manifold leaks but we are sort of saying we have ruled everything out on that side ........ hopefully !!
Cam timing has a big effect on manifold vac so we need to check cam timing ....i know this is a task but has to be addressed .
Ok ...... so we only have cam sensors on inlet cams by the looks of your data , but what ever you do to exhaust cam that will effect inlet cam as it will move also via the internal chain drive .
Them timing figures .......... on the pd diesels that kw figure is called torsion value and its normally runs as close to 0 kw as possible , i have just posted them timing figures on my other forum where there is folk with way more knowledge than me so i shall see what comes back ...........do the cams have a variable adjustment by chain tension or oil pressure .