No. It's just a card. It will run at the speed of the camera.
And, no, you didn't overbuy. Again: it's just a card. It has a large capacity and a high speed. If you never need the large capacity or the high speed, it will function perfectly at the capacity and speed you use.
From my personal standpoint, I use 32 GB Sandisk cards in all my cameras right now. I have no real need for the higher speed and I've never even filled up the 2 GB cards that I used years ago.