Pete, I pulled out my new Jaguar and set out to build it but have no history of downgrading cars. I looked everywhere to find out how to remove that supercharger and I never saw the option. Perhaps I'm missing something or just daft, either way if you could give a hint I'd be happy to do the rest of the build work.
Your not being daft Fit, You need to do a little messing about to take it off as its a glitch in the game, T10 is aware of it but has no plans to patch it unlike with the transmission glitches. I wanted to test it out and see if it made any advantage or just allowed different build options. So far i've not found any advantage, i've run the XKR a couple of times and I don't expect it to challange the other cars on these tracks. The Ford GT i'm trying in B class is a little better though.
Here's the steps to remove the supercharger.. As far as I know it works on any car with 1 other option in the aspiration upgrade menu.
1) Go to the Aspiration Upgrade and buy the change of upgrade.
2) Go to the Engine Upgrade and buy the SPORT level turbo / supercharger upgrade
3) Sell the upgrade straight back to the game, the Pi shown in the lower right corner of the screen will have dropped.
One side effect of doing this is you can then use the intercooler as ballast, you can still add it but it gives no increase in power.
My 300ZX is quite different to the other cars i'm trying out, It was originally built for a 'Bang for Bucks' challange and as you said its quite grippy to start with so I went with adding power with a cam and turbo upgrades to get as much Pi as possible for the 30,000 budget. This build has just added a few other parts to bring it up to the top of class. After running it today I want to see if I can rebuild it a bit though, it has plenty of power but feels a bit laboured in putting it to the road.