Webber dual carburetors = bigger pistons???

First, may we ask what Webers were you able to buy? 40 mm or 45 mm? Did they come with the manifold and throttle linkages? Bolting on larger dual carbs is just one aspect of engine modification. You might see better top end performance but if your driving is confined mostly to city travel as opposed to interstates, you might just see increased fuel consumption. To fully maximize the contribution of large side draft carbs, you have to go in deeper such as ported heads, larger valves, high compression pistons etc.