For quickness and ease of installation, I'd say go for the GEE fenders. These fenders will install a lot quicker than making your own set of fenders from Balsa. Practice making Balsa fenders in the meantime.
Regarding track time differences, there is more to it than just the fenders causing differences in times. It's one of the nuances of building these cars and it can have you pulling your hair out trying to figure it out. Most of all, enjoy the fun, and frustration, you'll have with these pinewood cars.