In practice you would need a mate to gun for you, and after a few manoeuvres he would no longer be your mate, because he would be shouting at you to turn left, turn right, dammit, are you blind etc. There would be a substantial but very transient novelty factor, and everyone would fly the Defiant at least once, but there are many more pressing aircraft that could be added, such as the _______ ___________ (insert). It's one of those aircraft that people make allowances for.
In theory you could monster Ju88s, if they were alone. I believe this was the notion back when the Defiant was specified. But in practice you would need to be directed to the Ju88s from a great distance by ground radar, unless you are prepared to orbit behind the front line. Mushroom's book gives the Defiant 1 a top speed of 304mph at 17K and a time to 15,750ft of 8.5 minutes, with its 1,030hp Merlin III engine.
A turret-less Defiant might be another thing. Boulton-Paul's original concept model was of a four-cannon fighter that looks the spitting image of early Mustangs, but it was not to be. I suspect that even the optimum conventional Defiant derivative would be very mediocre by any standards. It would have been tricky to work out where to put the guns, and to compensate for the altered weight and centre of gravity.