Yup...he's right - exhaust manifolds are very badly designed
You can get a few more horses by dumping the intermediate silencers too
Next step is twin throttle body conversion on each bank (like lister
) and say hello to another 40-50 odd bhp
(Although you'll need to add fuel enrichment - cheap way is to re-comission the disconnected/removed cold start injectors with a supplemental ECU, or remap it instead)
Then we have the GM TH400 3 speed, XJR-S valve block, or a B&M fast road shift kit to and get even better response. Or better still a getrag 5 speed manual conversion and knock a second of your 0-60!
I'd better stop there, I know far too much about XJ-S's that I'd be here 'till the small hours