At 38 dB you are at about 9,500 feet from the DSLAM. As Demonfang
mentioned margin is a bit low but not all that bad considering speed. Tx power is maxed out so what you are seeing is your DSL is syncing at maximum possible rate so unless you are able to improve the circuit somehow.
Be interesting to see if stats are any better at the NID.
DSL needs a filter to protect non-DSL devices from the high frequencies used by DSL. As you surmised either inline filters are whole house splitter will do the job. As long long as you install it correctly it can't do any harm.
If you are a normal Verizon DSL customer using PPPoE and ATM the total overhead TCP/IP/PPPoE/ATM is 14.4% so best case speed test report is 5.095 Mbps dwn/ .74 Mbps up. If you were really getting speed test results of 5.5 Mbps it means either the speed test is wrong, or you sync speed at the time you ran the test was higher then you posted.
/tom