Also, seems to be that the percentage of haywire missiles depends on the mass of the target. I haven't encountered a single case of missile-go-crazy on targets larger than 1-1.5K mass (haven't done a complete scaling test, just used my own designs of mass ranges roughly incremented by 500 each ship) So yea you'll have a miss chance on Isanths (and they are hard to hit with missiles even if they track anyway, because the large turn radius and tracking algorhytm makes it hard for them to hit small moving targets) but they are still among your best bets on hitting large targets (custom pirates and other player ships)
For hitting small ships, I suggest beams for easier targeting, even though they work best in the same sector as the target. A partial (5-10%) stop effect makes it even easier.