The gemara says that for one whose Toraso is umnaso, he wouldn't need to stop learning for davening but would need to for krias shema. Rashi says that the distinction is that davening is derabanan and krias shema is deoraisa. Does the heter of learning if toraso umnaso apply to any mitzva derabanan or only to davening?
The Shulchan Aruch (OC 106:2) cites this ruling word for word. The Mishnah Berurah comments that one who is learning Torah is obligated to stop to fulfill any Mitzvah, even a Mitzvah mid'Rabanan, and this applies even to somebody who learns full-time, since someone who learns and does not carry out what he learns "is better off having not been born." It is only the Mitzvah of Tefilah, which is "merely" praying for mercy, from which he is Patur.
B'Virchas Kol Tuv,