on the nasal breathing front... though i'm not sure if there is much research to back it up, it has often been cited to me from a number of coaches in various sports. the benefit i have found is that it introduces more control to breathing. in particular when doing short fast burts, and then then lower tempos between, switching to nasal breathing i find aids my recovery, and my heart rate returns to a lower level much quicker than if i continued with oronasal breathing. the same applies to hill sections too. in terms of volume of air inhaled, oronasal is well documented to increase overall flow in general, but this is not always the case, and some studies have shown that in submaximal exercise subjects do not always switch to oronasal breathing. in truth, it is probably down to the indivdual as to whether deep regular nasal breathing will work for you.