Well, since apparently everyone thinks raw fish is digusting, I guess that means more for me then!
(Seriously, people...not even lox?)
Although I wouldn't want to gainsay a vet with a Master's in nutrition, I feel it's fine if the OP feeds his cat sashimi. Most restaurant sashimi is pre-frozen anyway, so risks of parasites or whatever would be pretty small, one would think. I've fed my kitty pieces of salmon sashimi (frozen, then de-thawed) on numerous occasions, and he's been fine. No vomiting, no diarrhea, nothing. (But then again, he has a cast-iron constitution.) Tuna sashimi is even safer.
So I say go for it. I'd rather my kitty gets his Omega-3s from a natural fish source than from supplements.