A couple of other things to consider: the people that really need the low latency tend to use something other than Pro Tools, which if you browse here a bit you may get a sense of it performing not quite as good as say Cubendo. I think [with eg., an orchestra loaded] if you want latency down to where you're playing everything in and it's mostly useful, you want something else.
The last even small symphonic scale thing I got done, the buffers were at 1024; I don't manage projects so that there is a 'composing' template vs a 'mixing' template, though. I like the final sound to be what I'm working with composing; however I will go for instances with a lot less instruments loaded during early getting parts to track where I need to make drums happen reliably on time.
I demo'd MIR Pro thoroughly, I wasn't going to be able to fully exploit it without quite a bit more horsepower. There I would tend to want to dedicate the resources to it more after-the-fact. I don't even know how much horsepower I would need to stick every instrument in there while composing.
I don't know, but I tend to think expecting SSD to make things wildly more achievable through itself could be misleading.