Engines - artillery is a "fleeting target"?
Any ac that is on CAP (CAS) is far better able to respond than one sat on GCAS. Weapon loadout is always a factor, but none are better equipped for the whole range of targets than the GR4.
Ground Alert is a good backup to airborne alert, not a replacement.
We'd all like to have a carrier or 2 off the coast with GR9's & FA2's on board doing their thing, but it ain't going to happen.