I haven't compared it to an Evo, but RGTFanatic talked about the Gravity Storms, and compared them a little with other Vai pickups earlier today:
Originally Posted by RGTFanatic
The GS bridge is a LOT more cutting and "High-enddy" than a Breed bridge. I know that differs from the DiMarzio site, but it's true. The Breed is a lot warmer than the GS. I think this thing falls right between the Breed and EVO on an EQ scale, if there were such a thing. Oddly enough the GS neck has a ceramic mag. It sounds a lot like a hot-rodded PAF Pro to me, but again, because of the high-mids it's less forgiving of "finger-fuddles" or other mistakes. It DOES lack the "harmonics" of an AN. In that I mean, The AN in my eagle will fall off into a harmonic tone after holding a note and giving it a little trill. The GS won't do it no matter how hard I try. The difference could easily be the guitars though.
Also, check out the review on http://iheartguitarblog.com
. Peter (a member here as well as a great music journalist) made some comparisons to the Evos too.