Here is some interesting stuff I haven't seen posted yet, if it has, I appologize. This is the type of modded 454 TBI va454ss is using, also the person who directed me to the info. It is bored to 2.2". It obviously took some work, details at http://www.454ss.com/gallery/JimD/JimD_TBI.htm .
Now if indeed the holley 670s are rated at 1.5hg as a couple of posts have indicated, then I would imagine this 2.2" version w/ trimmed throttle shaft would have to flow somewhere in the 720-750cfm range. Even though the modding is a bit pricey, it still half the cost of a new 4bbl tbi, and there is no external driver board needed.
I am also more than confident that the injectors are capable of enough fuel flow given the correct pressure and fuel pump.

I have pondered boring mine out, but not quite sure I'm up for installing new sleeves and what not. I would imagine that you could bore it to 2.1" or maybe even 2 1/8" without needing to do that. The problem is that the walls are very thin to begin with.

 

In contrast, here is a standard 2" Holley base. The pic is from Brian's (Z28boy) page

 

Its my understanding that the Holley unit can't be modified.

Also, some of the GM 454 TB's can't be bored out either, due to thinner castings.

I received this info from Jim D., the guy that does the sleeving.