I bought a 7wt to fish streamers this year. While I will use it on my Kayak, I do not think that is as important as the size of fly you will be casting. A 5wt and large streamers does not work well for me. I have a friend that owns a fly shop and I told him I wanted a 6 for streamers, he strongly recommended going to the 7, so I did so on his advice. (Scott A3, 9', 7wt). Just rigged it up yesterday, so I can not tell you how it is yet.
Nice thing about the Yak. I use Scott rod holders. I keep two rods rigged and ready to go. One is in a holder, one between my feet. So you are able to throw streamers, then a dry, a popper and a streamer, etc on two different rods. It is very convenient.