if you're going to run the OS off the SSD then you have no need of ISRT. the whole point of ISRT is to use a cheap SSD (let's say 16GB) entirely as cache to speed up the OS as it runs off a mechanical HDD. it's a way of getting near SSD performance without having to spend tons on a sizable SSD.
Thanks for answering Branden!!! Just to make it clear, are there no benefits from running the OS in a SSD and running the cache using ISRT? Or there are some but not worth the effort because they are minimal?