I am as knowledgeable and interested in missiles as anybody, but I think the FAF nuke is fine as it is. The main problem with parabolic trajectories is that shots are hard to see when they fly that high, as in the case of T3 artillery and Mavors. The current nuke behavior is theatric rather than true to life. The same is true for the slow decent of the missile over its target. In real life, nuclear warheads approach targets at speeds exceeding mach 5. In Faf terms, that's almost a beam weapon.
Then there's the balance problem: Nukes are balanced against bases, because nukes are slow, but bases can't move. Obviously, a realistic fast nuke would change quite a lot about balance and would greatly favor ACU snipes.
Tldr: it's not realistic as it is, but it's fine