I think the target power ever since the 2.0 fixed class was bought in (ever since the RS500s were outed) has been 300hp.
That's a nice achievable figure with a 2.0l NA engine, but doing it with the help of a turbo means you can achieve that figure with a much less advanced (read cheaper) engine.
Also, beyond peak power, a turbo engine will give you a much larger torque spread (a broader, more usable range of power) which is perhaps why the turbos have enjoyed the advantage so far this season.
The mid-late 90s Honda's and Renault's were pushing 320hp+ NA, very impressive, but also very 'peaky' power, and extremely expensive. Using the same capacity with a turbo allows a much cruder (relatively) engine, and gives much flatter power. The regulators have achieved more reliable engines, that cost less, and produce better power spread, so its a +++, once everybody has it...
I have to admit, I've not kept up with the season thus far in 2011, so I'm not 100% up to speed