The A340 is certainly not known for being a rocket, it has a power loading of 4.39 (lbs/lbs st - 271,000kgs - CFM56-5C4). In comparison, the 772 has a power loading of 3.64 (656,000 lbs/297,824 kgs - PW4090). A 744 has a power loading of 3.85 (875,000lbs/396,895kgs - PW4056). For the Boeings, the figures will change depending on engine type. The A340's engines are much of a muchness. On the A340-300E, a 5% thrust bump is available for takeoff.
Whether it is underpowered, certainly compared to its competitors it is not laden with much power. From what I've been told by a CX captain (also flies the 333 with RRs, flew the 744 and the L1011, and 733 with AN), for the job, it is well balanced. Its main task is for long haul jobs and it does those very economically (uses less fuel than 777, but has two extra engines to worry about - maintenance). It may cruise slower than the Boeings, but that's a function of wing design. Compared to what a lot of people say, it's not that much slower, IMHO. M0.02 is not much of a difference, it's only noticeable because the sectors it flies are so damned long. Extra power will improve climb performance.
Even with a single engine failure, the A340 will still have a better power loading than a comparable twin.