Everything is relative. The A343 has a thrust-to-weight ratio similar to the B707, but less than the B744. The aircraft is certified to fly, then, it, of course, can fly fine. OTOH, if the lack of power was not a concern, tell me why Airbus decided to use an engine that is 2/3 more powerful than the previous one while the takeoff weight of the A345/6 increases by only 1/3 over the A343? The more powerful the engine, the heavier the engine and the more fuel it will consume. So, if it's not necessary, you don't want oversize you engine. Airbus learned from their A343 experience and decided to use a relatively more powerful engine on the A345/6.