Assault Boats
I think it's respectable indeed. Ion cannons can "only" do 1 damage, but frankly a single damage off a focused 3-dice attack is all you normally count on, and if you can ionize a target you can set up range 1 shots with the primary weapons pretty easily.
5 ships gives you a firing line that's hard to avoid and the sheer speed of XG-1 gunboats is amazing - especially now with Advanced SLAM so you're not throwing unmodified dice at the other end.
You could alternatively use Fire Control System but I think Advanced SLAM is the right call. On a list of 5 tough ships, focus is the better general-purpose action, and you shouldn't be needing to SLAM at high speed multiple turns in a row that often.