Out of curiosity, how big a group? Because this is IMO a genuine weakness of 4e -PCs synergise with each other so seven PCs is more than 40% stronger than five.
I can vouch for this, especially in Epic (running level 23). They just recently battled against three demon lords (lvl 28, 30, and 31 solos) and accompanying demons (2 marilith, 2 khlurich, and 2 balor) and have survived 8 rounds with only 2 player deaths thus far. This is with me working the angles and extending damage/effects into Fourthcore levels (minus the outright save vs death). That said, I did allow a kind of massive sacrifice/excellent rp to banish 1 of the lords, but that's the give and take of DMing.
I guess my main point is Epic requires, in my eyes, a lot from a DM that isn't simply baked into the game's regular encounter design (never mind my running 7 through the tier). All the MM3 math fixes fall flat in my eyes at this point in the game, given the ability to heal, the great interrupts, and the generally high defenses the party can attain and/or share. The framework of encounter design can still work, though, so long as I boost the damage and homebrew a lot of various effects/terrains/hazards/diseases. Auras are my good friend in Epic, too.
I've also begun doing something quite dastardly, but with great affect, in Epic characters having to make choices. They may be demigods in power, but not even they can be everywhere at once. I provide them with many avenues and enemies, and they have to make hard choices who to hit or who to defend, invariably allowing other enemies to gain footholds or enact their plots. It's the great gray area that balances mighty deeds, the idea of being so powerful you now must choose who lives and dies. Each of their decisions, even meant with the best of intentions or the most efficient of logic, still leaves millions out in the cold at any given time. It's been fascinating to watch their responsibilities grow with their power, and in some cases their guilt. I also tend to move Epic characters' agendas counter to one another, as I think would be realistic, which creates some party tensions, Survivor-like alliances, and more than one player facade. It's incredible fun, but I don't think any of it came from official encounter design...
You also need to command the pacing and support a kind of larger-than-life air to Epic, introduce plots and encounters that go so far beyond the scope of Heroic and Paragon it's nearly a different game. Or that's what I've found, anyway. Truth be told, I've lost players over the years from Heroic to Epic based on scope alone. They'd gone as far as they liked and didn't want to push it to the limits we're at now, basically fighting gods, the servants of the Great Old Ones, alien armadas, etc.