That should be correct - AFAIK - FileVault (1 or 2) does not have many extra (if any at all) writes vs an 'open' file system ... and this would also (as others have said) remove the need to ensure a full secure erase of the entire drive ...
Also Disk Utily gives you the option to overwrite the data once or more times ... over writing the data once (if that is in fact what is doing) is fairly good ensurance that some snooper wont pull up your data with a simple program.
SRM also allows you to specify approximately what type of data overwrite occurs.. IIRC you have a simle (one random pass), medium (3 passes - one writes 0s, one writes 1s, and one is random) or the sandard 7 pass (DoD - spec for standard HDDs) ... and maybe even a 35 pass option that would probably take a week on most full HDDs.
The multipass is more important (but probably less so if it is simply peronsal finacial type data) since they can read weak singals coming off the heads (and maybe make something out of it ... even after the data is overwritten) ... though I think this is pretty expensive, time consuming and complex - since they are reading the analog signal off the drive heads... not the 1s & 0s the drive would have interpreted based on that signal.