Not really studied the differences in more detail but I think that the "taking longer to dry hair" wont necessarily balance out the energy costs of high-energy dryers and low-energy ones. Hair can be dried quite well by just using a fan with a relatively modest heat, and fans use much less energy than heating elements so there could be some saving -- if I remember I will try to check the numbers to back this up.
Because this is an EU-wide proposal it could have an impact that while small is still noticeable. Roughly speaking the EU's energy use is comparable to the US and China (though still smaller, I think), meaning that reducing energy use across the EU will make some difference to global energy consumption.
In the long run it might make no difference after all, but I think it's worth a try. Even if it doesn't benefit the climate, which is what I'd be most concerned about, policies like this could reduce our personal energy bills (until the prices go up ridiculously high, anyway).