It was the JP5a which had them. The 5s at Finningley used for Nav training didn't.
The JP5 had the strakes. The 5 became the 5A after the avionics refit that introduced ILS and VOR/DME in place of Rebecca. IIRC the IFF equipment was also part of the upgrade. The entire nose section of the 5 was different from the 3/4 with a wider under surface and a more ovoid shape. The best explanation I heard for the addition of the strakes was to reduce the Cl of the forward fuselage and to maintain it at a more constant value under varying conditions. One can also speculate about the inertial effects but it is difficult to arrive at firm conclusions.
I had little to do with the 5B as flown at Finningley, but I seem to recall that the strakes were removed on 3 grounds:
a. Less drag - therefore lower fuel burn (see post above)
b. Limited use for spinning and aerobatics
c. Not usually flown by student pilots
The fitting of the tip tanks also altered the B/A ratio. I don't have figures for the 5 series similar figures for the 3/4 series with fuel in the tips and empty were .48 - 1.52. The comparative figures for the 5A/5B would be of the same order.
It is also be worth considering the effect of the fuselage cross section on spinning characteristics. Although the rear fuselage is most important as a contribution to the "damping in yaw" effect e.g.
Circular - 1
Rectangular - 2.5
Elliptical - 3.5
(all anti-spin effect)
You can see that the change to the forward fuselage would also alter the spin characteristics. I suppose that the bottom line is the fewer "bolt on bits" the better job the designer did!
YS