So my cheapo loop bars...
Similar shape to current bars, very slightly wider.
Mounted in stem, plenty room for hands in the loop, tape to mark where I was going to cut.
I tried weighting the bars after making the first cut - when the cut ends were very close and deflection would have been most noticeable - and I didn't see anything that worried me so went ahead and chopped more out. Probably took just over an inch out of the front of them.
Which it turns out was a bit pointless as mounting the thumbshifter on the 'extension' put it too far away from the grip to be useful.
I tried sliding bar ends onto the extensions to make a pseudo aero bar type thing but with nowhere to put my elbows/forearms I seemed to succeed only in creating an uncomfortable position.
My hands fit fine on the extensions, even with the trimming though so I'm going to persevere with the bars.
In other news, I also received an Ortlieb seat pack from eBay today and I'm very happy with it.