On balance, is it moderating the temperature? Between acid rain killing vegetation coral reefs, is sulfur in the atmosphere a net positive?

Depends on who you are and thus what is important. Farmers generally consider no acid rain good, even though they now have to buy sulfur fertilizer for their fields, where acid rain added plenty before for free.

Sulfur dioxide in the troposphere isn’t very long lived and won’t make it to the stratosphere where it actually has a surface cooling effect. Even small volcanos have very little impact on stratospheric aerosol formation since they do not inject sulfur to high enough altitudes. Only very powerful surface injections have long lasting stratospheric effects.

Organisations aiming to reduce sulpfur in ship fuel are defiitly aware of the cooling impact it has on the climate. However, the health benefits of this reduction are not to be understated - with shipping sulfur being responsible for 100s of thousands of premature deaths and millions of cases of childhood astham annually [1].

The cooling effect from ship pollution is there, but it is a relatively small fraction of the ovrall cooling from atmospheric aerosols.

[1] - Sofiev et al, Cleaner fuels for ships provide public health benefits with climate tradeoffs - http://dx.doi.org/10.1038/s41467-017-02774-9