I think I should leave that up to Fergal to answer, however its my understanding that what T::S does is different from what T::D does (I know for sure that the latter is considerably more flexible and attempts to solve problems that T::S doesnt and never will).
Test::Struct was simply my attempt at making an is_deeply() that worked the way I expect it to. To save you reference to some very long and nasty perl-qa threads I'll simply say that as the author of the most accurate Perl Data serialization module currently on CPAN I found every other tool for testing insufficient for my purposes. Test::Struct is/was a prototype I wrote for the kind of testing I wished had been around for Data::Dump::Streamer. Some argue that this level of accuracy is only really useful for testing serialization modules but whatever. Anyway i wrote T::S just before i went on holiday (which im still on) so its in an alpha state right now, somethiong i plan to resolve when i return.