yes, pick a set of cogs. 0.64 is very doable. There's threads on that, too, and the price isn't very high either. You won't get it down to 1500rpm, though, without going six-speed IMO.
That's this thread:
http://www.a2-freun.de/forum/showthread.php?t=23805 - and I'll translate one of the last posts:
"31/48 for 0,645. Under 45mph now no longer possible in fifth. 1400U = ca. 50mph, and this is find. Mine is unchipped. At 105 (tacho, ca. 60mph), he now has exactly 2k on the tachometer. No real change in "pull", slightly lower consumption to be expected."
The cogs came from Rosenkranz, €160, and the fitting was €122 incl. new oil.
Skinny tyres alone do little. Light wheels *and* eco tyres will make the real, noticeable difference, especially in comparison to the petrol-eating 17s. My difference between 185 winters and 215 summers is a full 0.7l/100, so €1 per 60 miles.
Bret