Well, under the Texel rating the Spitfire is rated at 104 to 101 of the F18's, that is not a large difference. I think the SCHRS rating is 1.035 to 1.005 for the F18 also three points. Pretty much, under both systems, the Spitfire has 80 seconds extra after 45 minute racing. That may sound like alot but it really isn't. They are just very good sailors, as alot of spitfire crews tend to be in my opinion.
Here Sproat/Burke sailing on the first day in the Texel Dutch Open 2007.
Wouter