Faster is better ! ....
Your device may or may not benefit , but at least a faster card wont slow it down ...
A good quality Class 10 card ( 45mb/s ) or faster wont hurt in any shape or form .
I have seen a lot of folks complain about their GPS lagging , or locking up , or not being able to keep up ... ??
Cant analyze the cause from here , but a cheap slow card could be at fault .... ( Could )
The other problem could be trying to run more demanding software on a older slower double din or GPS unit ..

What are you going to do ? ( Buy the best you want to afford )