RA III's are still D-port heads, and as far as port flow goes they're all about the same. They differ in valve sizes and whether or not they have screw-in studs. They've got the same "big valves" (2.11 intake, 1.77 exhaust) as all the other big valve, screw-in stud heads, so no - in my opinion it's not worth paying a premium for RA III's over, say, a set of #62's. They aren't going to perform any better. The best factory heads for Pontiacs are going to be any of the "round port" heads. Starting with 68 1/2 Ram Air II (round port exhaust, but same intake ports as the D-port heads), then the Ram Air IV's (round port exhaust, "good" intake ports), '71 455-HO (round port exhaust, good intakes, larger chambers), and 73-4 455 SD. The 455-HO's don't flow as well as the Ram Air IV's because the larger chamber volume "squeezes" the ports and hurts flow. This problem was addressed in the 455 SD heads, generally regarded as the best flowing factory heads Pontiac made --- with the possible exception of the super-rare and never-officially-in-production Ram Air V's. (those were completely different on both the intake and exhaust side, requiring different intake and exhaust manifolds/headers to match the ports). If you're able to find any factory Pontiac round-port heads at all, get your check book out - they're going to be super expensive.
However, if what you care about is performance, then none of them perform as well as just about any of the aluminum aftermarket heads, especially if they've been ported by someone who knows what they're doing.
Also, I have to respectfully disagree with 66 Gas Tires Oil on one point. Reputable builders do use Eagle parts. That's an old wives tale that got started in part due to a comment in the first versions of Jim Hand's book. Some of the very first Eagle cranks did have a problem in the shape of the fillets on the journals, but that was corrected years ago. I'm running an Eagle rotating assembly in my engine and it's fine. In fact, it probably saved my engine when the transmission let go at the track awhile back and let the engine spin to the moon before I could reel it in. If I'd had factory rods in this engine, I'd have been out much more than the cost of a transmission rebuild.