Normally increased boost does not influence tickover, I rather think that the standard airbox is getting the tickover rpm a bit down compared to the free flow filter, maybe a bit too low.

Boost increase on a standard, non chipped car doesn't do anything to the fuel supply. It is a mechanical system that does not recognise the increased demand for fuel when under higher boost. So be carefull, you could be running lean when under the increased boost!! And we all know what that could mean.

As for the cold start: the S1 has no choke, it has a cold start injector which makes the mixture richer when the engine is cold. The cold start injector only works at the moment the starter motor is running (key in position 3) and the engine is cold (below 10 degr C). So when the engine starts and you let go of the key, the cold start injector stops. Check the temperature sensor.

Cheers.