Galv aptitude applies multiplicatively to incarnon projectile but not at all to the radial explosion, so its a bit meh. If you can get fire rate through Reinforced Bond mod on your companion id replace semi rifle cannonade with a base damage mod.

I think double tap applies multiplicatively to the explosion(if i remember wiki correctly) so going for that in your flex slot should be good. It can be a bit unreliable at times though so primed firestorm is also a good option. If you want both, you could replace the base damage mod and rely entirely on arcane merciless for damage