There is no magic number when it comes to multiplexing, it all depends upon environment you have. Multiplexing factor of 6 may be a good number, when you have fast drives but slow clients. Whereas multiplexing factor 4 may be bad when you have slow drives and fast clients. It all depends upon what resources you have and what are your requirements.
You didn't miss any fast restore button. Under normal circumstances your restore will never be faster than your backup speed (assuming backup and restore are performed using same infrastructure and same topolgy with same kind of server/network load).
Check your bptm logs and see if it's complaining about buffers. Also, look at your HBA card setting, you may have a misconfigured HBA card causing slow restores.
I had asked you earlier also, was this backup performed using the same setup? If backup was performed using a media server and now you are trying to use local tape drives, you will have to make some changes to config to make it work.
Run a DD or tar or some other utility to see what kind of speed are you getting for read/write using these devices. Please make sure you have scratch tapes to do this kind of testing.
