The decreased frame rate has nothing to do with the new 2.0 firmware update. The 7D has behaved this way since its introduction.
As Neuro has pointed out, the 7D will decrease its frame rate in half, in low light in order to aid in metering. It will do this in AV, TV and Manual mode. In AV and TV if you lock exposure with AE Lock it will maintain 8fps. For some strange reason, in manual mode and auto ISO or fixed ISO, which technically it is locking exposure, the 7D will still cut its frame rate in half in low light.
There are 3 workarounds to maintain 8fps:
1) using AE* lock in AV and TV
2) using live view
3) pressing the DOF preview button simultaneously while shooting in manual
Unfortunately, none of these techniques are too practical or beneficial, because generally if the light is so low that the frame rate is cut in half, then your exposures will probably be too dark anyway. However, I have experienced this frame rate drop at dusk while shooting a bird in flight, flying low over the water of a dark lake, and occasionally with night sports in poorly lit portions of the filed, like the end zone.
This was one of the main reasons that I upgraded to the MKIV, as I wasn't sure if the 7D was intentionally crippled by Canon.
So Adam, it's not your lens, however slower lenses in low light will accentuate the problem.