The Phantom series never could go to 70, not without a tail wind anyway. Top speed pre new firmware was 45, can't say as to now though, but I have also heard that it's lower.
No, nothing will happen, as long as your firmware matches between your drone and he controller you should be good, the message is just letting you know new firmware is available.
I would recommend that you fly and turn on the drone without an active web connection though as DJI has a hardon for enforcing their new dynamic NFZs and I wouldn't put it past them to somehow force an update. I am assuming you are still using the pre "update" version of Go4 and have not logged in to accept the new terms of use yet.