I'd just get the proper shim set, particularly with butterfly bars which put your hands behind the clamp, meaning you have to have some very good clamping force to keep the bar from swivelling down when you hit a pothole. Speaking of which, have you tried butterfly bars? They put your hands closer to your knees, many people don't like them once they get to try them.
edit: upon looking at your photo, that actually should be all 25.4mm stuffs already. If you're measuring it at 26mm somewhere, it probably got ovalized.