What Josh said about the ported GT500 heads is true. I can not think of anyone who went to the GT500 head that did not make more power. You need to be circumspect when you do these sorts of evaluations because I also can not think of anyone who did not upgrade other aspects of the engine when they went to the GT500 Head so you will have difficulty doing an evenhanded A/B comparison.
What is absolutely certain however is that Ford made engineering changes to the basic GT500 head around 2012 or 2013 (don't remember which anymore) that substantially improved their strength especially when used in high power builds north of 1500 hp. The upshot was improved casting life at the elevated power levels and improved strength especially when using large head studs like the Accufab half inch studs with the 9/16 block side threads.
The very best ported GT500 heads have been reported to flow somewhere between 340 and some as high as 350 cfm. This
might be significant at high boost levels but at lower boost levels - not so much. For example if we look at engine airflow in four different engines, one is n/a, one is 15 psi boost, one is 30 psi boost and one is 45 psi boost we see very different engine appetites for air. The numbers look like this;
Now lets say you have a head that flows 320 cfm (at 28" of H[SUB]2[/SUB]O) that works out to a flow capacity of 320 cfm per cylinder times 8 cylinders or 2560 cfm. To help put these numbers into perspective, a pressure drop (or delta pressure) of 28 inches of water across a port equates to 1 PSI pressure differential across the port being flowed. The chart above shows the n/a engine's air flow appetite as 591 cfm at 8500 rpm. A cylinder head that is capable of 320 cfm at a delta pressure of 1 psi through a single port will have a more than adequate flow capacity for a naturally aspirated engine that only requires 591 cfm for the entire engine at 8500 rpm.
Yes you say but that is n/a. Well, when you go up scale and run higher boost pressures you get a scaled representation of the same phenomena. The heads again far exceed the engine's appetite for air. When you change the pressure drop across the port (boost) to 15 / 30 / or 40 psi from 1 psi, you are magnifying the port flow capacity by a country mile and you are back to the obvious question, why am I doing this and what was it that I was worried about again?
Net, net bottom line, in real world flow considerations the performance difference between two heads that are separated by 10 or even 20 cfm at the mid 300 cfm flow range will turn out to be negligible, if measurable at all, in engine performance. The associated changes that most engine builders make when they go to the marginally higher flowing head are typically the contributors to the increased power they observe and report.
That said, if you were going to build a race only or predominately race only engine at high boost the last generation of GT500 head is by far the most robust casting Ford has offered and worth the investment just from a parts longevity perspective - if you consistently race the engine at boost levels above 30 psi. If you are like most of us and usually operate well below 20 psi with the occasional foray into the 30+ psi boost range then a well ported "C" head is what you should be looking for. The monies saved will easily find other pieces to buy for the build.
When you are attempting to select a porting service pay attention to their mid and low lift flow performances. A robust flow performance from 0.200" lift through 0.400" translates into significant horsepower because of increased cylinder filling efficiency. Your valve is only at max lift once. It is at mid lift twice! Once during the opening ramp motion and once during the closing ramp motion. This is substantially more time than you will be spending at max lift. It is also where the bulk of the cylinder filling occurs.
The second place in your lift / flow map you want to pay attention to is the no-mans land between zero lift and 0.100" lift. The long duration camshaft opening and closing events are designed to take advantage of this cylinder filling opportunity just after the valve opens and just before the valve closes to both enhance the initial filling event and also cram that last bit of fuel air charge into the cylinder at the end of the closing event. If your ported heads under perform in this region you are basically chopping off "X" number of degrees of valve open duration - which begs the question, why did you buy that nasty longer duration, bad mannered cam?
Porting shops have had the ability to get much better at what they do in terms of port development with the advent of CNC port development tools. The best example if this is the Coyote heads. Both their intake and exhaust valves are substantially shorter than a corresponding Modmotor head yet the casting flows as well as the better ported modmotor heads. Why? Flow dynamics and port development learned over the last 15 to 20 years of CNC port development.
In general you will be well served by buying a ported "C" head done by a shop that produces robust low and mid lift performance - that's where the gold is buried. If you miss the max lift flow of your favorite internet killer head, in the big picture it is all but irrelevant. Go for the gold. You will find it is buried in the low lift and mid lift flow numbers.
Ed