Here' s my comments for what they' re worth.
I think petrol weighs about 0.7kg per litre, so a full tank would be about 45 kg.
With this over the front axle it would make a difference to the balance.
How long it makes a difference for depends on how fast you use it up [
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There was a review of the C4S in the March edition of Car magazine, but they made no mention of understeer (I doubt MY2003 is any different).
I would imagine it is possible to get more understeer with 17" wheels, compared to 18" , since the sidewall is taller.
The only other thing I can think of is whether the PSM trickery can influence it?
Tyre pressures will be the main influencing factor on a track since the tyres generate more heat than on the road. If the tyres are cold they will be under pressure and you would expect more work to be done by the shoulders and hence slide wide, otherwise if too hard the wear will be in the centre (looking at the tyre wear and feathering of the blocks will give you a clue).
A Boxster may be less susceptible to this.
There' s not enough info to form a firm opinion as to the cause, since it seems not everyone' s experience is the same.
Rgds, Stuart.