We're starting to get to this point ourselves - we're a relatively casual once a week guild (3.5 hours on a Thursday evening), that has historically been a 10-man, made up of a bunch of people that have been playing together for years now. In previous expansions, we've been edging onto Heroic modes by normally the second raid, and half to 3/4 clearing Heroics on the final raid. This expansion, we were off to a slow start (as per usual), and really only started raiding proper at the start of the year. Took a couple of weeks to clear Normal, and have been blazing through Heroic (probably thanks to the gear boost from Garrisons and normal clears) - up to 5/7 last night (with one-shots on all, including 2 new bosses), but stopped before we got to the penultimate boss (lack of physical DPS was a pain ).
We're running with our core of about 11-12 raiders from previous expansions, and have boosted that to about 15-16 or so with some fellow guildies, but I do worry whether we're going to get to clearing heroic before we get up to 20 players. Our server (EU-Burning Blade) is predominantly Horde, and Czech & Slovakian, which causes problems for recruitment. We were kind of hoping we'd be as crap as normal, and not get to mythic before the next raid tier comes out - I fear this may not be true this time around!
This all being said, I like the idea of 20-person raiding being static for Mythic modes - gives the designers a lot more manoeuvrability in raid design (assuming there exists one of each class, etc) - we just need to get recruiting!