Not everyone can verify it. You have to verify it by it's chassis number. If you want I can help.
The fuel consumption isn't that big of a difference. The biggest difference will be 2-3Km/L. The performance isn't improved in either case, it's the same. The only difference is the transmission the 2WD has the K410 Super CVT-i gearbox whereas the 4WD has the conventional 4-speed automatic gearbox.
Sorry, but you can't compare the two. They both are in different categories. The Belta is in the sub-compact category whereas the Tiida is in the compact vehicle category.
Some direct competitors to the Belta are the Honda City, Kia Rio and the Chevrolet Aveo.
The job of a catalytic converter is to create harmful gases such as carbon monoxide into less harmful gases such as carbon dioxide and H20 (water) which don't harm the enviroment and the ozone layer.
Have you scanned the car with an OBD-II scanner? Have you checked the fuel pump and fuel filter?
Don't remove the CAT converter, they don't clog up easily because immense amounts of heat is passed through them. The Belta has three CAT converters like all modern day vehicles. One is right below the manifold, the other is the mid-pipe and the last one is in the muffler itself.
The guys at 'Toyota', they aren't even experienced. A guy who does a basic check-up on his at home knows much more. When the hell does a CAT converter block dust? What the hell?