Hello,
I just checked in the lab that once you put the interface in bgroup, you can not change the bandwidth of assigned interface.
Only bgroup related bandwidth can be changed.
e.g. if I put eth0/4 & eth0/5 on SSG140 into bgroup0 & eth0/6 & eth0/7 in bgroup1, I only get option to select bandwidth for bgroup0 or bgroup1. I do not get that option for eth0/4 or eth0/5 or eth0/6 or eth0/7. So I think manually putting two difference bandwidths on two ports of same bgroup is not possible.
However all ports eth0/4 to eth0/7 can be kept with auto negotiation mode & full duplex settings & peer interfaces can be configured with different speeds to check if interfaces part of same bgroup can auto negotiate different speeds based on peer interface settings or not. You can give it a try though I have doubts about it.
Regards,
Rushi