There are three factors here.
The first is that compared to the competition Hyundai's power output is competitive. The Beta is competitive at 138hp with the:
Ford Focus SE (136hp)
Honda Civic (140hp)
Nissan Sentra (126hp)
Toyota Corolla CE (126hp)
Mitsubishi Lancer ES (120hp)
Volkswagen Golf GL (115hp)
The Alpha's 110hp is competitive with:
Scion xA and xB (103hp)
Toyota Echo (108hp)
Toyota Yaris (106hp)
Honda Fit (109hp)
Even close to the Golf GL's 115hp
The problem comes from comparing Hyundai's offerings to non competing products. We would all like to see Hyundai compete directly with the Civic SI, but at this time there is no competetor from Hyundai so you're comparing apples and oranges.
Second point is that even though Hyundai's engines are currently competitive the introduction of the Theta and Lambda engines will further raise the Hyundai Performance bar to the point of competing directly with the hotter versions of some of the cars mentioned above and many other cars that are currently not direct competitors with Hyundai. The Theta is slated to have a 300hp turbocharged variant in the upcoming Dodge Caliber SRT-4, and there is a rumored 400+hp version of the EVO also sporting a Theta. With this potent engine in Hyundai's arsenal it would be surprising if we don't see a turbocharged variant producing at least 200hp find it's way into the Elantra, and maybe even into an Accent similar to the Accent SR. The next Tiburon is going to be rwd and possibly awd and powered by the 3.8l Lambda producing over 250hp naturally aspirated. With these two engines being introduced into Hyundai's product line there will no longer be any reason to clain Hyundais are underpowered.
Last point is that Hyundai's Alpha, Beta, Delta, Sigma, and Sirius engines are not weak, they are simply detuned to add reliability without requiring extensive overengineering or pushing the 100,000 mile warranty out of a reasonable price range company wide. They do utilize fairly dated engine technology, and are slowly going to all be replaced with stronger, more powerful, more efficient versions like the Theta and Lambda. Detuning is a common practice in the auto industry, a good example is the 5.0l found in my Bronco. It is rated at 185 horsepower, while the 5.0l in a Mustang the same year is rated at 225hp, 40hp more. You can expect a longer service life out of the Bronco's engine as a direct result of the lower power output. Many military vehicles have even more drastic detuning, like the Unimog S404, equipped with a Mercedes-Benz 2.2l and 2.8l I-6's rated at between 80hp and 110hp while the versions in Mercedes cars were outputting up to more than 180hp. MB did this because in exchange for the lower power output the Unimogs' engines were now virtually indestructable. It was nearly impossible to blow one up because when working at full load the engine only saw stresses comparable to light acceleration stresses in the automobile versions of the engines.
Well, those are my $0.02