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Between the narrow beam it produces, and the fact that it is not always well centered on your field of view, I think you would have problems with almost all portrait situations. If you really are shooting at 400mm or more, it might work, but it is a bit hard to even interact well with your subject when you get that far away.
You also have a big issue with the light being very close to the camera and lens for the normal way a Better Beamer is mounted, which in general gives poor lighting on the subject, and also will make some really strong redeye. I highly recommend you get your flash a bit away from the camera, up some and perhaps also to one side, to get a better lighting effect. You might still be able to aim a Better Beamer on to your subject if its beam has enough coverage, but the aiming will be critical and take time.
Also, consider a second flash well off camera as your main light, and then one perhaps still mounted on the camera but a bit up as a fill flash. The second flash will add more light, and adjusting its beam size using the built in focusing reflector to match your lens field of view will give you the best use of the power available.