Putting a lot of weight (200kg or so) in the boot of a calibra turbo in front wheel drive makes them corner better , doing the same to my 1.8 vectra or my merc 180 has the opposite effect ( 200kg in the boot of vectra is virtualy undrivable )

We better tell the F1 teams that they're doing it all wrong then. They need to load their cars with lead in every conceiveable space thenn, so they handle better in corners.

What are you talking about? A formula 1 car is rear engined rear wheel drive meaning the engine weight is over the driving wheels! And yes formula 1 cars are very light meaning they are virtually un drivable at slow speed due no down force pushing the car into the road. Even though a formula 1 car is light it's force against the Tarmac gets bigger the faster it go's due to down force pushing it harder into the road, it almost gets heavier the faster it go's (in a way)

Your taking weight from the front of a front wheel drive car and putting it in the boot? How is this going to help front end grip? Please explain because you've lost me.

And your comment about formula 1 cars is completely irrelevant as we are talking about a front engined, front wheel drive car. 

Threads like this are bad.

I agree, removing weight from the front end of a front wheel drive Calibra is going to make the problem worse! Less weight means less grip means you put your foot down mid corner the car will go straight on. Simple science.

Simple, at least!

For best cornering 'grip' you want ALL the tyres to be evenly loaded. Obviously, this is not possible, so you want to reduce load (weight) transfer as having one heavily loaded tyre and one lightl loaded tyre at the same end has less total grip than the evenly loaded tyres.
This is why anti roll bars are used to help the cornering balance - by using a stronger ARB on the rear, the outer tyre is even more heavily loaded and the total grip is further reduced - the ultimate is with the inside wheel just lifting off the surface - any more is detrimental as the body roll increases positive camber on all wheels and reduces total grip.
With front wheel drive cars, there is the complication of power applications reducing grip on that end (traction/grip circle concept) and the desire to have as high a proportion of mass (weight) as possible for maximum power transfer to the ground.

So, a rough suggestion would be a combination of..
LSD if inside wheelspin is a problem, together with throttle control.
Use springs (with dampers) and ARBs to reduce ride height (lowered CoG and reduced body roll), provide suitable bump travel and allow the rear to lift no more than just off the ground (may not need to be that stiff, depending on vehicle).
Play around with tyre pressures, these make a big difference.
If looking at weight saving for a track/play car, I'd look at initially removing any excess parts, like seats, and replacing the glass with plastic as that will remove high weight which will improve CoG height, body roll, performance everywhere and as most of it is towards the back, it should help the balance of the car.
Some may want to play with cambers, caster, toe settings, tyre sizes, etc, which is a whole new ball game.

Forgot, tyres DO make a big difference, check out what the other guys are using, I'd recommend a spare set of rims for the track tyres, so you don't chew them out in general driving.