GPS data logger as a lap timer

Wondering if anyone has experience in using GPS loggers or has an idea what the realistic obtainable accuracy is on these sort of devices.

google brought up:
http://www.gps4less.co.uk/shop001/product.php?xProd=2462&xSec=1
http://www.easydevices.co.uk/pp/GPS_Receivers/Bluetooth_GPS/GLOBALSAT_BT-335_BLUETOOTH_GPS_WITH_DATA_LOGGER.html

I worked out which if a device could give your position to the nearest 10m then the maximum error in laptime would be +/- 0.45s assuming an average speed of 100mph. To use the data I intend to write software that can produce lap and split time plus other stuff like average speed etc.

Just a toy for a trackdays seeing you'll get kicked off for using beacons and not every track has magnetic strips plus I'm not really interested in having any sort of live output / LCD display.