They have always kept their cards very close to their chest. Outside of peak, and since its inception, I can count on maybe two appendages (digitally-equipped ones, that is!) the number of times the load called for a -300. During peak is another matter. But either way, much of the actual "load" is box-enclosed sailboat fuel.
The service was kicked off with immediate access to a bunch of -200s, and the Prime-painted -300s trickled on as they became/become available. Even though most of it can be done with -200s most of the year, the very presence of a still-growing fleet of painted -300s pretty much points to a plan for this to be around for a while, with room to grow. Will it eclipse brown or purple? Probably not any time soon, or at any sort of rate that would put either of them in jeopardy, beyond slower potential growth (absent Prime) versus the economy.