I haven't tried it yet, but normally, the valet mode on new cars from the factory locks you out of accessing any kind of stored phone numbers, addresses, garage openers and such. I don't believe these make any changes to power or driveability.
I know on my Audi's APR tune modes you can load a valet tune which limits boost and top speed, but thats different then the Valet mode you can lock out in the glove box.