[RC-Monster Mike]

A 50mm Medusa is a little on the small side for the Esavage - but it isn't THAT far out of whack really if properly set up and used. I use a 2000kv 60mm Medusa in my ERevo (a couple pounds heavier than the ESavage) - I also know how to set up my car and check temps, etc.. The E-savage is like a slightly larger 4wd stampede in size - noticeably smaller than an emaxx or ERevo. Tha savage flux is a different story.
I think the real message is that if the non-Castle approved motor fails and takes the esc with it, it isn't covered under warranty. The 50mm Medusa is on the small side of ideal for that application, and is therefore more "prone' to failure vs. a more appropriate motor selection. I think the motor would fail long before the esc in this instance, though a failed motor can take the esc down with it(depends on how it fails).