[Graham Boak]

To fulfill an earlier promise:
The removable weight of the eight Brownings and 2,876 rounds is 405 lbs.
The removable weight of four Hispanos and 362 rounds is 754 lbs.

So the effect of the cannon would be almost twice that of the machine gun fit.

The positioning of the weapons in the wings would indeed affect handling rather than performance, but the extra weight would affect the climb and acceleration, although not the top speed. It is 2/3 the weight of carrying a pair of 250lb bombs, where the effect on handling etc. is perhaps more easily recognised.

Inertia roll coupling can occur when large masses are offset from the aircraft centreline. The extreme case is of course something like the He162, where the offset is asymmetric. In the Spitfire/Hurricane's case the masses are symmetric; but any yaw and roll, as might well appear in combat, would introduce asymmetries. I'm not strong on stability and control (never was, I was a performance man) but I suspect you are right that the effect would show as a Dutch Roll. Not welcome when a stable firing platform is required.