Hi Mark,
Badly set up webers "burn a shit load more fuel"
Well set up webers can be "reasonably" economical
Triple carbs of the weber variety (multiple stage jetting) allow more precise metering of the fuel at high flow rates than carbs such as SU's etc which have a variable jet. Plus triple webers are effectively 1 carb per cylinder (3 x pairs) so a greater volume of air can be throughput. Bolting on a set without other mods will do little on its own but used in conjunction with say a nice ported head, fast cam and decent exhaust then good HP gains and top end performance will be possible.
Its all about using components to complement each other, not individually.