Besides Viral Damage and status chance, obviously. I've been slowly wringing all the MR out of my favorite primer, my Kuva Nukor, and I've gotten to the point where I'm not sure if my mod selections are quite worth the slot... and I've got plenty of capacity to work with.
This is my current build. To me the point is that it applies max stacks of Viral ASAP, as an added bonus it's Kuva element is Radiation which prevents buffing enemies from doing much of anything. As is unless I'm on SP the Nukor can and will kill most enemies quite efficiently.
So, what do you do with a Viral Primer once it primes Viral with all the gusto is can?
Edit: Taking all the suggestions here's what I've got:
Couple suggestions: Eject Magazine to make it reload when you have your melee out, Embedded Catalyzer for extra status chance, heat element so you can slow roast something if melee doesn't work. Secondary Encumber is a good choice too.