If it’s on, select “manage storage” in “storage & backup” and you’ll see your devices. Select the device with the Sonic Racing data and scroll down the list to “Sonic Racing”. If it’s marked as “on” and both the iPad and the iPod are on the same account and are both using the iCloud, they should be sharing the same game save. I don’t have more than one device, so you might want to test it by unlocking something on the iPad (like a cheap track or racer) and then see if the iPod syncs and updates the game.

I’ll shoot a message to the dev to see if they can answer your question as well.