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Not sure about wasabi but soy sauce is the topic of a lot of debate. Ferris himself says it's ok in his personal blog. I use tamari (gluten free soy sauce) on a fairly regular basis and it has no effect on my weight loss. You should try it and see what happens. You can always cut it out if it has a negative effects.
Wasabi should be OK--the paste you get in sushi restaurants is usually powdered wasabi (a root similar to horseradish) mixed with water to make a paste. . .there may be some starch as a binding agent.
Ask your favorite sushi restaurant if they have fresh wasabi--it's SOOO much better anyways!
As for soy sauce--It will likely make you gain weight, but probably not gain fat. Soy Sauce is ridiculously high in sodium, which will cause you to hold on to water, as your body attempts to maintain proper sodium concentration in the blood.