Well, the synergy is in the devour effects of the Scarabs that RTing the Mummy ultimately generates with the Boneyards/Walls of Death. I will admit Earth/Darkness gets more out of it than Time/Death did, because there was already existing synergies that worked well.
Yes, like Blood said, the upgrade could very likely have 7 attack, since +3/+3 is around what vanilla creatures usually get for upgrades (Not dragons ofc)
+3/+3, really? Horned Frog only gets +2/+0, Cockatrice gets +1/+1, Graviton Mercenary gets no bonus, Hematite Golem gets +2/+3, Flesh & Phase Spiders get +3/+0. The only creature in Elements that gets +3/+3 upon upgrade is Blue Crawler, from 3|3 to 6|6, and it used to not be that way (Abyss Crawler was 5|5 in all versions prior to 1.21). So I would hardly say that +3/+3 is a typical upgrade for a vanilla creature. I could see the upgrade with 7 attack, though. It seems reasonable.
Yeah, I failed there. I was actually only looking at the Abyss Crawler when I said that before I went over to my calling bed, but I did consider the +1 cost it gets when you upgrade it, which basically pays for the 3rd +1/+1. With that in mind, I can now say that on average vanilla creatures will get around +2/+2, with +0/+2=+1/+0. Also, adding cost generally gives about +1/+1.
That too, was just speculation.