Depends on who you walk all over. Do it for someone with good connections and they can do some pretty bad things to your reputation and chances. Do it to a psychopath and you can loose your teeth or something else of importance. Do it to a lawyer and you can loose your house/car/momma/or whatever you have. LOL
There's so many mixtapes out there that it's most likely no one that matters will ever hear yours. But in the offbeat chance you get a record deal with a beat you have stolen, and the owner of the beat won't give you permission to use it, you're ****ed. And there are producers who will just out of spite or principal deny you rights, 'cause they are pissed you stole it instead of asking. I personally would tell the record company I have a 10 times better song written to the beat than yours and offer it to some other artist on their roster and tell you to **** off.
If stealing gets you to the door crack, don't expect people to be happy about you.
+ Mixtapes are so last year anyways. Do 1-3 hits and you have better chances than with 10 mediocre songs.