SRT isn't as fast as Fusion with normal usage, since it has a limited SSD component and doesn't adapt to your usage over time. It simply caches the small amount of storage you recently used more than once. Fusion will keep your entire environment of repeatedly used apps, data, and library files on the SSD for fast use, and does so with way better granularity than you could possibly do with symlinks.
Of course, but SRT has been shown in benchmarks to be very nearly as good as single drive SSD (which is what I was saying), and theres no way fusion is faster than that, so likely SRT is nearly the speed of fusion.