CFM matters if the line is constricted. I haven't tried the PC pancake but I've experienced OEM quick connects that were so constricted impacts would refuse to run until replaced with a quality connect. What's more the Porter Cable pancakes come with 1/4" hose while all impact guns I know of require 3/8 hose with 1/4" NPT fittings. Not a big deal but if the OP decided to go air after all, a new hose would be needed.
From what you are saying, it sounds like you aren't quite properly grasping the relation between CFM and power. This is a common misunderstanding. The CFM of the pump doesn't have that effect on the actual amount of air flowing through your hard lines and air hoses as you think. Compressor CFM is only the rate that the pump can put air into the tank, not the rate that the tank can put air into the your lines. In fact, when your compressor isn't running, the effective CFM rating of the compressor is 0 CFM. If CFM directly determined power (in all cases), then whenever your compressor isn't running, you would have no power at all with your air tools.
Even the most restrictive air fittings (1/4) flow at the rate of about 30 CFM, which is far more CFM than even the most air thirsty 1/2 drive impact needs (usually running CFM for these guys are around 24-28 CFM). The reason people then get better results with higher flow fittings is because more restrictive fittings lower the PSI of the air going to the gun. Less restrictive fittings (or a higher initial PSI) enable you to run the gun at full power. This is how it is possible to run a heavy duty 1/2 drive impact off a 20 gallon compressor for short bursts (and still achieve full power) and take off lug nuts from trucks, or heavy duty suspension bolts.
CFM, again, only comes into play when you are trying to use an air tool for a long period of time, as CFM ratings (on compressors) is better understood at the rate in which the compressor can recover/maintain a certain PSI than it is the rate at which air flows out of the compressor and to the tool.
Another way to say this is, that the CFM rating of your accessories (air hose, fittings, etc) do matter for the power of your impact (because of their effect on lowering PSI, which reduces the power the gun operates at), but the CFM rating of the compressor does not influence power unless you are using a continuous use tool (which would drain the tank below the desired working PSI, reducing power). I've run impacts of a 4 gallon pancake before. You can still get full power for brief uses, but you do have to wait for the tank to refill in between uses to get that full power.
PSI determines the power of the tool
CFM determines the compressors ability to maintain power for long duration, but it does not directly determine the power of the tool
I think I said that all right. It's a bit confusing at first, but you'll get it!
TO THE OP. Get a harbor freight EARTHQUAKE Pneumatic impact gun. 3/8 (300 ft-lbs) or 1/2 drive (700 ft-lbs) will have enough power for almost all tasks on a light unibody vehicle. Since you didn't mention any big requirements (other than it just kind has to work), I'd go for the smaller, lighter 3/8 drive model.
The pancake compressor (2 gallons is it?) will be enough probably for a few seconds of use (5 lug nuts or something?) though on any impact. A slightly larger tank or compressor would be a good idea too. You don't need to spend huge with this. I bought (for $50) a 28 gallon craftsman from the 90s. Works great for all of my impact wrenches (although it is LOUD).
