Hi Harmonica - I had the T2i, and you're right. Like the GH3, it only goes up to ISO 6400, while later Rebels go up to 12800. That said, I shot some ISO 6400 at 50mbps IPB and 1080/60 with the GH3 and the slow 14-140 kit lens outdoors the other night and it looked terrific.
I didn't shoot this, but here is the GH3 and the slow 14-42 kit lens from ISO 1600 to 2500 at 72mbps ALL-I:
http://vimeo.com/55708059
I don't know whether it is the bit rate or something else, but this camera produces great images in low light - even though its max ISO is "only" 6400.
Interesting that the shooter from the video above says in the notes:
"I have been shooting Canon T2i's and a 5D Mark II but they both are being listed on Craigslist"
To answer your original question, I would not buy new EF lenses if you plan to buy the GH3. They are a challenge to adapt because the aperture defaults to wide open when they're powered down.
You either have to buy an
adapter with a manual aperture built in, or an expensive powered adapter.
I would wait and buy either micro 4/3 mount or Nikon mount lenses with manual aperture control.
It is easy to find inexpensive
Canon or
Panasonic adapters for manual Nikon lenses.
Given that the
T5i and
SL1 just came out a couple of months ago - I don't think we'll see an upgraded Rebel any time soon. Canon doesn't seem to be interested in either 1080/60p or higher bit rates for their Rebel cameras.
Cheers,
Bill