Regarding cogs - I'm pretty sure Shimano dished cogs, which aren't all that scarce, have the same spline pattern as the Sturmey hub. I considered using a larger cog instead of smaller chainring, but then I thought I'd have to do more fiddling with the chainguard if I put a larger cog on. A larger cog will increase the distance between the chain and the chainstay - I'm not sure if the stock mounting hardware for the chainguard can adjust far enough to compensate for this additional distance. Using a smaller chainring reduces the distance between the chain and the chainstay, so there's no problem with adjusting the chainguard.
I like to spin more than mash, and definitely wouldn't have used 5th gear with the stock gearing unless heading down a steep hill, so I'm pretty happy to now have a full usable range of gearing with the smaller chainring. Harris Cyclery has Origin-8 130/110 BCD chainrings (formerly "Rocket Rings") at a budget price if you can't find anything at your LBS.
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