If you "double" a creature's power more than once in a turn, each doubling effect applies independently
It also asks a couple of questions of deckbuilders considering it: "How much do you want a bunch of 1/1 Insects, and are you willing to play mono-green to get them?" Looking at the available data, the answer seems to be, "not all that often." That is, unless you're running an Insect kindred Commander deck with Zask, Skittering Swarmlord, Xira, the Golden Sting, or Vrestin, Menoptra Leader, in which case the answer is still just "sometimes." 14
The Purple-Blue Vinsmoke Reiju deck revolves around Vinsmoke Characters like Reiju, Niji, and Ichiji as key cards of its strategy
Many MtG mutate cards have abilities that trigger when they (or another card) mutates
On the other hand, if you are willing to increase the budget just a little bit, playing Path to Exile over Declaration in Stone would represent a big improvement (plus, if you're getting into Modern, Path to Exile is a card you want in your collection anyway since it's a true staple of the format)
Niv-Mizzet Parun draws you a card whenever any player casts an instant or sorcery spell and deals 1 damage to any target whenever you draw a card
