The key to a decent combat Keelback is a good crew member. Competent minimum better is better
You can train them pretty easily in a HIRes by following the cops and kill stealing from them. Until they get past competent I will dictate when/who they attack and put them on passive most of the time while I swap between the fighter and the KB.
My normal mode of ops would be jump in the fighter (Fixed beam Taipan for me) Set the crew to defensive then pick my target. Strip their shields and get them focused on me. Then get the crew in the KB to engage. If the target is a tough one I would often lose the fighter at which point I take over the KB and keep fighting (well setup with some engineering its a tough nut to crack) and when the fighter was rebuilt I'd launch it again with the crew on attack mode. I would be able to farm HIRes all day long as long as I had enough mats for reloads etc.
Overall I love the KB and if it weren't for my current foray into the bigger medium ships (Python and Dropship) I'd probably be in my Keelback.