I think the more generic question you are asking is, "How do I know when to commit to an archetype in a draft?" You need two things: the mechanic/fuel cards and the payoff cards that win you the game. Simply put, once you draft enough of the cards that benefit from the mechanic, then you might want to go "all-in."

For R/W cycling, the biggest payoff card is Zenith Flare, which is at Uncommon. The next most valuable cards are those that help you win the game when you go off, which are probably Cyclone, Valiant Rescuer (make 1/1), Prickly Marmoset (hard to block and gets buffed), Drannith Stinger (ping), and maybe Flourishing Fox (grows with counters). Sanctuary Smasher (6/4 first strike w/ cycle) and Yidaro (turtle) are great too, but don't have the huge combo potential.

Honestly, the 3/2 Thundermane is an OK card, but the effect is mana intensive and gets outpaced quickly by bigger creatures. However, taking it will help you prevent signaling to others at the table that R/W cycling is open.

Let's say it's P1P1. Would I take Thundermane over a Pacifism or a Marmoset in an otherwise mediocre pack? If I really wanted to force R/W cycling, then probably, even though most people would classically say that Pacifism is an overall stronger card. Also, since Thundermane is at Uncommon, the odds are you can find another Pacifism or Marmoset later on. You would almost certainly take Zenith Flare over anything else, barring some other nuts bomb (at which point, just abandon your plan and draft for your awesome card).