Looking at the ingredient lists for wasabi paste on Tesco.com, they seem to be made with some oil, but I would agree that it isn't something of which you are going to have great big spoonsful (unless you have a stronger constitution than mine!) so it probably won't make that much difference, syn-wise.
I sometimes have the sushi from Pret when I am in town and can't get home for lunch. Quick and easy and very nice indeed and not too bad for syns (better than anything else in Pret, anyway!).
When I worked in London I occasionally went to one of those restaurants where the sushi came round on little plates on a conveyor belt and you just grabbed what you fancied as it went by, then took the empty plates to the cash desk so they could work out how much you had eaten. It was great - but easy to eat FAR too much!!