We compared the line out from my 5G (using the dock, not a mini connector) to the Red Wine on a 4G. We went into a Grado HPA-1 (the original big black Grado amp) and into Senn 580s. It was very close. My friend felt the 4G sounded better, but there wasn't a major difference. I wasn't sure I could tell the difference at all, maybe with more extended listening I would have been able to, but it was so close it didn't matter to me.
I'm planning on comparing the line out via the connector adapter vs. the dock, but haven't gotten around to that yet (the dock allows you to change the volume from the remote, the adapter doesn't, so they could be different - or the dock's just smarter in how it talks to the iPod).
We'll probably repeat the test with Grados RS-1s and Sony 5000s as well, but I'm not expecting a major difference between the iPods even with those.
The Red Wine is a great mod on a 4G, since the 4G output electronics are very weak, but the 5G is a huge improvement in that area over a stock 4G and sounds great.
BTW - on the subject of 4G vs 5G, the headphone output of the 5G is also a massive improvement over a 4G - with UEs and Sony MDR-V6 cans we heard distortion on some solo piano through the 4G, but not the 5. Also, and this is really apparent with cans like V6s or Sporta Pros, there's much more headroom and punch through a 5Gs headphone compared to a 4G.