If so how much of an improvement? How much effect on insurance?
The FMU is just an AFPR, personally i’d say go for the BEGi S kit (it’s amazing value for money), but get your own fuelling and timing solution…
the thing with just using an AFPR is the fact that you have to retard the timing across the range, making for sluggish pickup at low revs, get a megasquirt or e-manage blue, and you have much etter control over the engine…resulting in better performance, better miles per gallon and an ability to push it up more than 5 or 6 psi (i’m running the full proper BEGi kit, at 11psi with an Emerald K3 ECU and my god it flies!!)
So good choice, just ditch the AFPR 
If you want more advice don’t hesitate to pm and if your not on mx5nutz already, then get over there! much more FI based knowledge over there…although there is a huge debate on BEGi v FM (because they don’t like mx5mad very much)