If you find HT speakers that are outclassed by computer speakers, it's probably because 80% of their cost is in the electronics that came with the HT rather than the speakers. In other words, these reviewers have some crappy HTIB setups, not real HT speakers. Besides, it's functionality the Z-680s don't really have, so it's an apples to oranges comparison in the end.
And Logitech hasn't improved upon their speaker technology since the Z-560s. They have just been rebadging their old speakers with new looks for the last 5 years, with a few added (non-speaker related) trinkets added on. So, no, their next "revision" is not going to be worth the price they charge if you already have one. It's the equivalent of buying a new version of the same $500 bookshelf speakers every few years, from "v1" to "v2" to "v3" and etc. If you sell the old it and just want to replace the looks and are willing to "rebuy" the same speakers for 50% of its value every few years, well no one is going to stop you.
As for thicker speaker wire, it's all about power loss through resistance. Thicker wire has less resistance, so less of the current is lost. In a small HT setup with speaker wire runs no more than say 15-30 feet, there is no empirical gain greater than 16 gauge.
You probably want to look at offerings from Creative and Klipsch if you want to "upgrade" in the PC speaker market. Creative and Klipsch both find themselves in the same rut as Logitech, rebadging their systems every few years, although their speakers were better to begin with. Klipsch has essentially done nothing in the last 7 years to update their satellite speakers. Creative had an "epiphany" and added a tweeter earlier this year. That's about it.
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