Giblets may be right but I have no experience of the 2.2. That said, IMO one of the issues with the lardy 2.4 is that the front springs are too soft, the spring rate is wrong too. This is the major cause of understeer in the car. So on that basis it's possible that the standard 2.4 Ti springs would work very well in the JTS model.
It's certainly worth experimenting, especially as what I'm selling isn't going to be expensive.
Yes they do when combined with a heavy engine and dampers with not enough low or high-speed compression. Obviously as as family road car the platform is designed to understeer a little. But it's far too much for my liking!
No. It's the opposite.
If you stiffen the rear anti roll bar the car will roll less and the softer front end will bite in. A softer front anti roll bar will have the same effect.
The springs will have a lesser effect on understeer but its' the same rules. Soften the front springs and it will bite in better. Or stiffen the rear springs.
What you are trying to get is the front tyres to do more work by moving more.
I always start with the anti roll bar settings. If you look at a well prepared racing saloon (FWD) you will see that it corners with one rear wheel off the ground. This is because the rear has been set up to be very stiff. Then the looser front end bites in more and you get less understeer.
Of course the general physics will dominate in a heavy diesel at the front so you won't 'cure' this. We set up cars to understeer in production because most drivers can cope with a car that is going straight on when going fast around a corner. There is a natural instinct to lift off the throttle. A car that oversteers gets worse when you lift off the throttle.
So, to cofirm. If you stiffen the front springs you will increase understeer.
Never argue with an engineer they say! I worked as a suspension tech at Silverstone for ten years so know a little about it. The physics you quote are spot-on but the practice is different IMO. I'm currently at Snetterton with the car in question, and I can assure you that I have far less understeer with my new set-up without a doubt. Come and take it for a spin if you're anywhere near and then we can discuss it over a pint in the bar later!