Hi Barton
Theory says there should be a power gain with the manifolds. I prefer to think of it as eliminating a power loss! Gain will be highest at high gas flow - ie high revs, and it is entirely dependent on the rest of the system being equally free. Biggest power gains on any engine, though, are to be had from making sure it's running right!
Personally, I am very happy with the noise inside the car, but this is a very personal judgement.
The louvered bonnet is an attempt to address underbonnet temperatures. Step 1 is of course to limit the heat generated in the first place - hence the discussion about exhaust wraps etc. There are a couple of health warnings with the louvers though. First, there is some suggestion that they allow underbonnet fumes to access the heater intake. I have no practical experience as yet, but Rover used the same solution on the P7 prototypes and this comment from the owner of P7/4. This car has the louvers somewhat further into the centre though (P7/1 had them in exactly the same position as I have). Second, they wern't very easy to get put in! We first had a go with letting in a panel a little larger than the louvers, but that wasn't very satisfactory. Eventually the entire bonnet took a trip to Aston Martin Restorations at Derby, who were the only people we could find with a fly press big enough to get the whole bonnet in.
And yes, louvres look very good in an S2 bonnet as well! My Thai car has this set up with a single centre scoop. If you pm me your e mail address I can send you some photos.
Regards
Chris