CHRIS,
There are not as many studies comparing TM with breathing techniques as comparing TM with techniques that were competitive in the second half of the last century, such as exercise, listening to music,
Jacobsen's progressive relaxation technique, and alpha wave biofeedback. I could be wrong, and I welcome contributions to this discussion by others. One study,
Transcendental Meditation, mindfulness, and longevity: An experimental study with the elderly. published in the Journal of Personality and Social Psychology (1989), found a longer lifespan, better cognitive flexibility, and lower blood pressure from TM, less so for mindfulness training, and still less for relaxation. However, it should be pointed out that this study was conducted by researchers biased toward the TM program.
A Google search using the terms "transcendental Meditation" "breathing" revealed studies showing the significant effect of TM on reducing breathing (which is a side effect of deep meditation), such as
Effect of transcendental meditation on breathing and respiratory control from the Journal of Applied Physiology (1984).
Transcending (as happens in TM and NSR Meditation) produces a measurably unique state of physiology, corresponding to a fourth major state of consciousness (in addition to waking, dreaming, and deep sleep). Practicing this state of consciousness for a few minutes a day has a vast beneficial influence on many areas of life. Unfortunately, there is little scientific or experiential evidence that breathing techniques (pranayama) or breathing awareness techniques (mindfulness) have as profound an influence, as compared to transcending. As far as I am aware, such comparatively superficial techniques don't bring the depth of refreshment,
measurable reduction in anxiety, and feelings of bliss associated with transcending for most practitioners.
David Spector
NSR Meditation/USA