Does this modem transmit at 0db in leased line mode?

No, the standard MT2834Bx has a max transmit level of about -9 dB. This was designed into the modem to maximize V.34 performance. If there is problem with a leased line, the provider of that line will often point to the fact that the modem is not transmitting at 0 dB and ask that the transmit level be raised before further troubleshooting the problem. Most often the problem is not the transmit level. With a modem transmitting at -9 dB, the loss on the line would have to be in the area of 34 dB to put us at the end of our receive sensitivity threshold of -43 dB. Most leased lines are spec'ed to have no more than 16 dB loss from end to end.