If I go back to my Plantronics Voyager Pro HD and take the same level of calls, I can easily go 4-5hrs and Bluetooth is no where on the top of the list of the battery usage. Also have more battery left over say like 40% still left over. My Voyager Pro HD only lasts about 3.5-4hrs now, battery isn't as good as before where it would go 6hrs solid. The Legend goes 7hrs but kills the phone in the process.
Something with how it seems the Legend connects to the S5 and causes it to use more power. I cant find specs on the Legend or Pro HD to see what Bluetooth levels they operate on.