No one has replied so I'll give you my $.02 and that is probably about what it's worth. First, as a former saltwater fisherman who had a Furuno LCD bottom machine I'll say that Furuno is about as good as it gets for "bottom" and fish displays in saltwater. How good the Humminbird will display in saltwater I just can't say but it is very good in the shallower water bass fishermen deal with.
Now, as to high speed display, I think it will display as fast as your transducer installation allows. Bear in mind that the xducer for si is somewhat larger than for just sonar alone so I think it will be a bit more difficult to install for high speed. Some of the other guys on here are divers who probably have experience closer to what you want to know than I do. I use mine strictly for si and for sonar while fishing and have another unit for high speed running. I hope this is some help and maybe it will at least trigger some discussion. In an ideal world you would have both, one sonar for higher speeds and si for structure location and to see the shape and size of the structure. Also, the coming bottom scan should be ideal for seeing just what you are fishing if you bottom fish, I would love to have had that when I fished for snapper and grouper.