Do I really need a 90-degree arm/torso angle?
That 90 degree suggestion is purely for getting an optimal fit for aerodynamics. By bending your arm to 90 degrees, you're not presenting any of your forearm to the wind. If you watch the pros ride, this is typically how they'll carry themselves on flats and descents. Ultimately, you need to be comfortable though, and if bending your arms that much causes you to lose that much power, then it's probably not the best position for you. It's possible that there are other modifications to your fit that could be done to correct this, but that would require experimentation and/or a professional fitting. Check out this video from BikeRadar. It addresses your question pretty well.