Angier,
   I have an stec 55x in my cessna.   I can tell you that even in direct-to mode, gpss is much more stable on this particular unit than straight nav. It may be must a peculiarity of the stec unit, but without gpss it's kind of wondering around the sky.
   I don't know how the -30 compares but I would suspect it's no better, in which case you will be sorry if you don't get gpss.

Colyn