can't hurt the speed, only help it.
your current card has been used a lot. NAND memory cells have limited life. the memory controller will remap away from dead cells, sure, but the longer you use the card, more chances of a bad write.
CF card prices are cheap. You don't even have to buy the highest speed, 1000x by Lexar, you can go for 600x, or even 400x. That way, you don't pay the premium for the current fastest.
higher capacity. more shots on the card. the minimum available is probably double the size of your current card.
gives you a chance to get rid of your sandisk cards and switch to Lexar. If you look at Rob Galbraith's CF database, Lexar occupies a majority of top ten spots. The database doesn't have the 5D classic, and the camera won't take advantage of UDMA cards, but... by getting a card faster than the camera can write, you are certain that the bottle neck is the camera and not the card.