45 deg laterals are frequently used in relief systems made from thin wall stainless. I agree that SIF's should be equal or lower to a 90 deg where the branch to header ratio is close to 1. However when the ratio is significant, big header, small branch, then local shell flexibility must be a major player. Looking at the acute angle crotch, a bending load will concentrate stresses at back of an un-controlled fillet weld. Not a good situation.

The only place I have seen laterals addressed in a national Code is the French Codeti, where tee sifs are similar to B31.3 but with a factor of
sin(a)^3/2 in the denominator, where (a) is the acute angle.

To establish sifs using finite elements, look at FE/Pipe software by the same author as Caesar II, the venerable Tony Paulin, at www.paulin.com.