I'm no engineer, but, it seems to me that your bars are going to flex LONG before your stem will (smaller diameter, force being applied farther out, etc). don't be so concerned about stem flex, get a fair priced, light one and call it a day. start at the bars if you're concerned about flex, after you get a good stiff bar then consider a stem if flex is still even noticeable. just my opinion and I'd be interested if this is an incorrect opinion as I don't ~really~ know