K, hypothetical example.
Rshaman
#1 and rshaman
#2 are both healers on alliance, in a 10v10 premade.
The opening midfight happens, and alliance wins the midfight, pushing it towards horde's gy. No one has died. We take a look at meters then.
Rsham
#1 has done 35k healing, with 5k overhealing.
Rsham
#2 has done 15k healing, with 20k overhealing.
Right there, we can see that Rsham
#1 can predict burst, pre-heal, and/or react faster than Rsham
#2 .
For overhealing specifically, it doesn't matter how much of the BG the two shamans are apart, because their ratio of healing to overhealing will always tell a lot, no matter how long you are near a healer.
For healing output in general, healing done isn't a great method of comparison, but it is still useful. Assuming that one resto shaman isn't left solo to defend a high stack FC while the other resto shaman is healing a midfight, then recount is going to be extremely accurate, as both shamans will generally be near each other, moving around the map with a midfight. I am not saying that one rsham might not fall 5-10k behind in healing because they broke off to catch a moving FC, but if a healer is being smashed in healing by a large margin by a healer of the same class in a premade setting, then it is a solid indicator of the skill of the two healers. Obviously there will be exceptions/other things to look at, such as one shaman might be dispelling/purging/shearing and playing more aggressively while the other shaman is pumping out heals, or one of the two disc priests might be pumping out frontline damage.
Also what % of a healer's total healing each spell makes up is a very important thing to look at. (at least for MW and disc, not as much for rshams, and no idea for hpally/rdruid, but they aren't used in premades for healing). If a mistweaver's enveloping mist is only consisting of 20% of their total healing, then they are doing something wrong, and they should probably look at their chi management for ways to improve their healing output. As I mentioned earlier, if a disc is doing 60-70% of their total healing with PW:shield, then they are just bubble spamming and wasting weakened soul debuffs when they are not needed, and should be flash healing and penancing more