sorry to say that like most things with the W12 unit then the engine has to be partially removed / dropped on its mounts to gain enough access to replace the cam sensors
a faulty cam sensor will prevent the variable cams from functioning correctly and they will remain locked in the 'safe' starting position - this results in a lack of performance and loud exhaust - as you have already noticed
if the 2nd throttle is also giving issues then it may not be opening fully either
this parrt at least is easy to access and you cna check it over by removing the air intake pipe and then with the ignition on and engine off, have your assistant slowly push down on the accelerator whilst you watch the throttle move
it should move steadily and smoothly with no jerkiness - if you spot this then 1s ttry cleaning the throttle and if that doesn't help then replace the throttle body
I'd also check the connections to the EGR valve, if either the vacuum pipe of electrical connection are damaged / corroded then the EGR can be on all time which certainly will hurt performance. Its also possible that its a fault with the EGR valve so its worth removing the valve and cleaning it out with brake cleaner, if it looks particularly clogged then replace it.
and it would also be worth measuring the fuel pressure at the injector rail with a running engine as this will show if the main engine pump is in good condition
as low pressure will limit power and engine rpm