I have never heard of the Spitfire being raced singlehanded at a serious event. I have sailed singlehanded in club races when the wind is force 3 or less and it really moves, but it is quite a handfull above that.
If you check www.schrs.com you will see that the Spitfire (139Kg)is lighter than the FX-One (153Kg). The Spitfire also has a larger mainsail area (15.45) than the FX-One (14.85. so, in the stronger winds, the Spitfire needs more weight to hold it down. In Ireland and the UK, where we regularly get force 4 to 6 winds, we would not be competitive solo in those winds.