This is what I found from experimenting.
If you use the card in a usb2 internal card reader you get write speeds which are fast (determined by the speed of the card and card reader ).
Im getting 3.6 MB/second write speed ( Windows 7x64 i3 internal card reader usb2)
If you use the card in the Garmin the write speed is determined by the firmware of the Garmin (I'm getting 800Kb/second write on windows 7x64 i3 usb2 nuvi 1300)
The card is Sandisk 4GB Micro SD HC.

I don't think that using a faster card would improve performance in the Nuvi.