At what point are either/both of these worthwhile:
I'm certainly going to be supported a critical mass of flashback soon enough.
Thanks for the input. I have plenty of food for thought!
Runic Repetition is basically never good enough in my opinion. Most flashback spells are either not very high-impact or just game winning by the second cast. I don't really want to re-cast my Firebolt for a 3rd and 4th time on turn 7 in %90 of matches. The few Beast Attack/Roar of the Wurm type cards that do exist (i.e. High impact enough to want to cast multiple times but low impact enough so that they don't just win the game most of the time) are few and far between. I'd rather just have a Divination in my deck over Runic Repetition most of the time.
Quiet Speculation on the other hand is a card I like quite a bit, depending upon the makeup of the format. It clearly signals that flashback is mechanically significant and playable in the format. It helps enable the (admittedly narrow) Burning Vengance deck by tutoring up 3 cards that go directly into the yard. Imaging grabbing three draw spells, a Beast Attack and Roar of the Wurm, or even just a couple of random spells that can be cast for cheap form the graveyard. There's a lot of possibilities with this card, but requires a huge mass of flashback to work. I'd say try it, but be careful!
If you end up going all in on Burning Vengeance, might I suggest going a little deeper on the Escape mechanic? I really like Satyr's Cunning as a role-player for both spells and red aggro strategies, since it can turn expired dorks, burn, and cantrips into a constant stream of creatures and spell cast triggers. Since the tokens can't block, they don't mess with races too badly.
Underworld Charger has really impressed me in THB limited. He's a 3/3 that comes back as a 5/5, which can be very hard for some decks to deal with, but in a fair way. It's always struck me as "strong, but not busted," and actually I've considered adding to my main cube on a couple of occasions.