You're gaining lots!
I seldom shoot at 2.8, even when using APS-C --- Even on APS-C, you often want to stop down to F4 to get adequate DOF and to sharpen up the lens.
For extreme low light situations, I'll keep a 1.4/1.8 prime in my bag, regardless of APS-C or full frame.
Part of the beauty of full frame is that you no longer need 2.8 is as many situations.
When I switched to full frame, the biggest improvement I saw was not in extreme low light situations -- it was in every day "medium" light situations --- Situations where I previously needed a 2.8 aperture to get an ISO 1600 shot... and had to keep the shutter speed a little too slow.....
So the shot might have some motion blur, might be soft from using the lens wide open, and DOF may be a bit too narrow..
To being able to use a faster shutter speed, getting to stop down the lens to it's best use, to getting adequate DOF..
Take a 50mm shot on APS-C, that I previously shot at ISO 1600, 2.8 aperture, 1/40 shutter speed....
Now take the same shot, at ISO 6400, 4 aperture, and 1/80 shutter speed --- The second shot was much better.
Shooting at 2.8 is not a "good" thing --- unless you are going for super narrow DOF. Instead, huge apertures are sometimes a necessity for low light. With full frame -- it's no longer necessary!
If the noise performance is the same, I'd much rather shoot at F4/ISO3200, than F2.8/1600, in most situations.
Take a look at:
http://www.photozone.de/canon-eos/425-canon_1755_28is_50d?start=1
The Canon 17-55 2.8 -- You really need to stop it down between 4 and 5.6 to control vignetting and get maximum sharpness.
For optimal sharpness.... since I'm not usually aiming to minimize DOF, I tend to stop down to F8 as long as I have adequate light.
So my shots, stopped down to F8, end up looking better on fullframe than on APS-C. (Not to mention, my long lenses weren't 2.8 anyway previously).
Not to mention improved dynamic range and color depth! It's not just ISO, though that's the biggest difference.
Certainly, some people over-estimate the utility of full frame. But I feel many APS-C shooters dismiss the idea of full frame without realizing the advantages.
I was an APS-C shooter for years. I didn't think I'd ever really benefit from full frame... As I contemplate switching systems now, I don't think I can give up the benefits I've seen with full frame.
1250 is still flawless on full frame. Shot at 2.8, the image would have been too soft. At 4.5, it's nice and sharp, with still plenty of bokeh.
My standard walk-around zoom is a 2.8, because I'm fortunate that Sony makes an economical 2.8 zoom, but if and when I switch to Canon or Nikon, I'll simply switch to a F4 zoom. As I rarely ever use my lens open wider than 2.8 anyway.
(I would still try to use a 2.8 for shooting indoor sports... a matter of getting every possible advantage. But if I was rarely shooting indoor sports, I'd probably just live without it).