Have a look at the root fairings on the DC-3 and Spitfire (to name only a couple examples). The DC3 had to climb to a specified altitude on one engine, and the Spitfire had to retain energy in an aerial battle. Both goals used the same thoughts: smooth out the airflow at the wing root, along with other facets. Both goals were met - as we now know.
Now, the fairings on those planes might be exaggerated a bit, tho I would think both were highly optimized for the task at hand. Those fillets had to be rather large.
The AR5 has smallish fillets, but Mike knew where the canopy had to be relative to the wing, so it seems his fillets ended up smaller than the dramatic parts on the transport and fighter.
I produced wing root fillets for the F1, but as far as I know, none were tested without those parts attached. I did sell some of those to other racers, but again I did not get any feedback regarding speed. One fella did say the plane felt more solid at lower speeds - but what does that mean regarding performance?
I also recall one fellow producing fillets that bulged as opposed to the std sort of fillet - seems I heard they worked, or did not - at about the same frequency.
So it seems you have to specify to yourself what the use of the part is to determine it's size, but it does seem to provide at minimum a better climb rate, or lower drag at high alpha angles.
If you proceed, please let us know what your opinion is after installing the parts.
"Not everyone needs a Rocket. Some folks, however, shouldn't live life without one.
You know who you are."
Budd Davisson, 1997