GNU tools won't unpack gzip'd tar files in a single step; I need to gunzip, then untar... probably just me

Nope ... same for me, too. Cygwin's tar utility will do it in one step. I haven't tried BSDtar.

Although Cygwin's tar seems fine for unpacking, it can sometimes pack in some unwanted attributes/permissions (all files marked executable, in the one particular case that I'm thinking of) - so I avoid it if possible.

Cheers,
Rob