Ivan,
I have not tried the Sony tape drive but I used the desktop backup on my SME 5.1.2 and restored it over a 5.5 fresh install and everything has been working fine, passwords and all.
I tested it on a developement server first. Just to make sure it would not hose anything. If you have the ability to test it before you do it on the production machine it may make you feel more at ease.
Tony