hmm...I´m not much into the new cards, but as far as I can see, heartbeat of spring is good old mana flare in green disguise. So if you want to splash red, you might consider playing manaflare instead. Or you might play both
If you can make 10 mana, you might be able to make 8 for biorhythm too. More life - more creatures -more life -more creatures...
The really tough task is to gain so many life that storm herd makes sense. Actually, you cant. But you can try. I believe that the best life-gain-engine I know is land tax + ivory tower. Sun droplet can work wonders too. And then, of course, there are different infinite-life-comboes. Maybe they thought of storm herd as a wincondition for life-decks. Its so overcosted. Which decks can make 10 mana and still have enough life to make this playable? Only decks with infinite-life-combo.
Death grasp and drain-life-typespells fits fine, since x-spells tends to be insane with heartbeat of spring and mana flare. But then, why make Pegasi, when you can drain?
Or you could make some combodeck which plays glacial chasm, and is able to make 10 mana fast. 20 pegasi turn 1 or 2 is nice. But I guess I could find a better wincondition.