Will a 300 Blackout upper work with the Super Lower?

Yes. 300 AAC Blackout does not place additional stress on the lower over 5.56 x 45. In fact, your gas system and buffer mass are much larger factors in how much stress is placed on the lower receiver. A heavy buffer with a lot of gas on a 5.56 gun will place more stress on the lower then a light buffer properly gassed on a 300 Blackout gun.