I am interested to identify the effect of replacing $\oplus$ with $\boxplus$ on basic balanced Feistel structures over $r$-rounds. Given:
$F_\boxplus[L,R]= [S,T] = [R,L \boxplus f(R)]$ where $f$ is a PRF
$[L,R] \in \left\{0,1\right\} ^{n} $
Q1 . In terms of CPA and KPA security, does $\boxplus$ replacement add security over 3 rounds, with proof?
Q2 . If $\boxplus$-Feistel structures have better security bounds than $\oplus$-Feistel, why are they not commonly used?