If you look at your std hand guard, you will see the first 5" is solid. This is the hottest part of your barrel, the chamber, and the next 2 or 3 inches. The plastic is not a conductor of heat, so I gotta believe, any full length vented would help to carry away heat. Now if your ripping off 20-30 rd clips at a time, I doubt one would notice the subtile diff.
Just came back from Delta force at the Houston gun show. They had Item CM070204, it was thick plastic, but has large vent slotts full length. Also Item RD1495 Made of thin steel, which had small slots over full length. Which one is best? I believe either is better than the std one.
If you buy from delta force, just know that according to posts, they don't want to give your money back if your send it back.