I already do fly the VIII, infact I'm thinking that it will be my main ride from now on. It would just be nice to have the correct historical options for ALL aircraft. I think it is a little foggy on whether the XVI had the extra tankage and I dont know about the XIV.
If the XVI had the rear tank, then so did the LF.Mk IX as they came off the same production line with the Mk only being determined when the engine, Merlin 66 for the LF.IX and Merlin 266 for the XVI, was installed.
OK, per Spitfire the History, there were tests with rear tanks, 74 gallons for the high back and 66 gallons for the low back versions.
That said, in the specifications on page 433 it describes the fuel thus:
Fuel. 100 octane; 150 for +25lb boost
Capacity fuselage (upper) 48 gals, lower 37. Total 85. Plus 45, 50, 90 and 170 gal o/ld tanks.
There is no mention of the rear tanks in the specifications.