@eos:
In addition to the information from Boki_Srb (Thx Mate):

If "overall performance" - you understand also "how many countries" includes the installed program - then the answer is "YES" - SD card capacity is important! (The map of each country is located on the SD card, so the card has more capacity - more maps will fit.
Other - As Boki_Srb wrote

regards wojpen