Is oxidative phosphorylation less efficient in gram positive bacteria?
A similar question was already answered at How do Gram + bacteria use a proton gradient for F-type ATPase? . In those answers, there is a general belief that Gram negative bacteria do not have much control over their intermembrane space pH, because their outer membranes have many porins, such as OmpC, which allows free passage both ways for protons and other molecules smaller than 600 daltons. If that is true, there is not much of a difference between Gram positive and Gram negative ability to control pH just outside the inner membrane.

There is a long review at DOI: 10.1016/j.bbamem.2005.09.010 . The first two examples of pH sensitivity are E. coli (divides more slowly when pH goes from 7 to 8.7) and B. subtilis (divides more slowly when pH goes from 7 to 8.5). The first is Gram-negative, the second is Gram-positive. B. subtilis undergoes growth arrested at pH 9, suggesting it might be more sensitive, but again, the difference is just subtle.