Hah! I forgot all about this thread.
I wound up widening the bars 2" (50mm) on each side for a total of four inches (100mm) overall.
The difference is not huge or anything surprising, but it very definitely makes the bike much handier at any speed below 50mph (80kmh).
All I did was section off two pieces of ordinary steel handlebar, cutting with a pipe cutter to ensure square ends (my chop box wasn't available).
I beveled the end that mates to the stock bar, installed the Oxfords and controls in the new location, drilling new locating pin holes two inches further out, and then clamped them on with the longer bar-end screws.
They've been in place for a good four years or more and easily 35,000 miles (56km). No issues whatsoever, not looseness, uneveness, nothing.
The extra leverage in real life is a pleasure in traffic and in driveway and parking situations.
I can't detect any negatives out on the open road, although I suppose it may cut an mph or two off top speed should I ever attempt that.
Easy to do, barely an hour, and a big help for managing the bike.