Sfeikos in Shiurim

"Kol shiurei chachamim l'hachmir chutz mk'gris" (chulin 65). Rashi learns this is the old rule of sfeika d'oraysa l'chumra, skeika derabbanan the kula, and gris is just an example of a derabbanan. However, Tos, would seem to argue. The gemara in 4th perek of brachos has a safek whether the zmanei tefila are ad v'ad bichlal or ad v'lo ad bchlal. Tosfos asks what the safek is - "kol shiurei chachamim l'hachmir" except by gris? The implication is that even by dinei derabbanan (like tefila) you would be machmir on a safeik in shiurim. Tosfos seems very strange - why should other sefeikos derabannan be l'kula, but a safek in shiur be l'chumra? My suggestion (until I think of something better): R' Yosef Engel has a chakira in Esvin D'Oraysa whether issurei d'oraysa are mitzvos hagavra or mitzvos hacheftza. Assuming that issurei d'oraysa are issurei cheftza, but issurei derabbanan are issurei gavra, one could understand the rule of sefeika d'oraysa l'chumra is really because we are machmir on a safeik in cheftza and not a safeik in the chovas gavra. By way of analogy, I recall R' H. Shachter once cited a Degel Reuvain to explain the reason according to the Rambam a safeik issur kareis is mechayeiv an asham taluy (when acc to Rambam sefeika d'oraysa is just an issur derabbanan) is because a safeik issur kareis is an issur cheftza while all other sefeikos are issurei gavra. Since shiurim serve to define the cheftza shel mitzva, even by an issur derabbanan one could argue that one should be machmir.