Ainarssems is right, +/- 0.3s is not very accurate. If you want accuracy best to look for a gps with sampling rate of 10 times per second, then accuracy should be +/- 0.1s
But, its good fun to be able to use VCDS, who wants to throw away £400 on a Performance box meter or similar.
http://www.seatcupra.net/forums/showthread.php?t=87905
the cheapest performance mod is to run with your fuel tank almost empty. If its a 90l capacity, you could drop around 60kg, which could improve your 0-60 by around 0.2s