This is a great example of considering the individual as well as research.
Also considering what the research actually tells us - for example, testing 6 ponies is completely understandable from a research point of view, and provides useful information, but is obviously not enough to draw conclusions about most horses from.
In this paper, grazing muzzles are found to have several benefits for the ponies tested, and no measured problems. For these ponies, grazing muzzles would seem to improve their welfare.
For the individual horse - consider the pros and cons and that horse's reaction to the muzzle. Although the ponies in this research showed no increase in physiological stress in the measurements taken, some horses are clearly very stressed by a grazing muzzle and there may be other less stressful solutions to weigh management.
Throughout nearly 20 years of looking after Paddy, the only item he has ever consistently avoided - with an added kick threat if I persisted - is a grazing muzzle. In his case, weight control was essential, but he seemed much happier with a small bare paddock and company. Others have accepted it without seeming too concerned. For the horse who is clearly not thrilled about the muzzle but does tolerate it - careful ongoing evaluation of your options!
Learn from the research, but always listen to your horse too.