205 GTi 2.1 TD - XUD11T

[Ruan]

So the risk is only to brake the alternator belt.

I run like that during more than 5000km before with my first R.I.P 2.1td (standard conrods broken).
And the belt is still alive.

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Do you have picture of the 406 hdi alternator?

The risk is to the belt, alternator and the dampener - it will be made much worse with the lighter pulley. You can usually tell the clutched, freewheeling alternators as they have a cap over the pulley like so:



It will also help the engine's response as the the alternator can spin down freely, so the engine revs fall faster, meaning faster gear changes etc.