For the non-ice staves, you give up 10 magic accuracy for about an 8.7% increase in unresisted damage. If your land rate (with HQ staff) is above the cap, this is win. If your land rate is at the cap, it still wins, by a little. As your land rate decreases, the margin becomes smaller. The crossover point is near 75% land rate, where an HQ staff will do about 0.857 of its "rated" damage, on average. A magian staff will do about 0.789 of its rated damage at 65% land rate, which (for a damage affinity +4 staff) is about 0.858 of the rated damage of an HQ staff.
For the Aquilo's/Varuna's comparison, though, you give up substantially more accuracy, and a little damage. You lose 22 to 25 magic accuracy, depending on conditions. The damage from the 5 INT should be around 1.5% of your total, give or take. Clearly, if your m.acc is well above the cap, Varuna's wins. If your land rate with HQ staff is just at the cap, however, you already lose. A capped accuracy staff will yield 0.974 of rated damage. Varuna's at 83% land rate will yield 0.986 of rated HQ staff damage, which will be very close to a capped Aquilo's with the +INT. That isn't nearly enough to cover the accuracy spread between the two staves unless you had surplus accuracy before the swap.
But you cannot consider these issues alone without looking at the bigger picture. First, we BLMs have easily capped accuracy on many mobs at level 80. For all of these, the magian staves win. Second, in the cases where we don't have capped accuracy in our max damage sets, there are other items that make sense to swap first. Swap your Zenith or Morrigan's hands for AF/AF+1, for example. Going from Morrigan's to AF costs you ~3% damage and gives you 10 m.acc. Combined with the transition from HQ to magian staff (non-ice), you break even on the accuracy and come out more than 5% ahead on damage.
Sure, you have to be a little more careful when you swap out your Aquilo's, but you still have a lot of other gear to swap before you can't come out with a net gain in damage by using Varuna's. There can still be some extreme cases where we might revert to HQ staves, but these will be the exception. I haven't fought Khimaira since the cap increase, but that is the type of scenario I'm picturing.
+Dmg affinity magian staves have too much damage bonus to ignore. Swap other pieces of gear to make up the accuracy penalty if needed, and you'll still come out ahead most of the time.