I also have the 5x Powermate and it has an even smaller field lens. You are not losing any light from the primary as all rays focus at the field stop. Not any different to a short focal length eyepiece with small field stop diameter. You’re still getting all the light that is available from the primary.
Think of the Powermate as part of the eyepiece. A 41mm Panoptic (68deg) has a 46mm field stop diameter. The 2” 4x Powermate makes it a 10.25mm (68 deg) Panoptic with an effective 46/4=11.5mm field stop diameter. The clear aperture of the 4x Powermate field lens is about 17mm (rough measurement) and this is well over 11.5mm needed for 10.25mm/68 deg eyepiece.
The 5x has an even smaller opening because it is higher amplification but too is only 1.25” so only needs to accommodate eyepieces with max. 27mm field stop diameters. 27/5=5.4mm. It’s clear aperture looks about 10mm (didn’t measure but well above 5.4mm).
You are not losing any light and the clear aperture of the Powermates is well above that needed for any eyepiece with maximum field stop diameter for a given barrel size. It is no different to using a shorter focal length eyepiece with smaller field stop diameter. Note the Powermates are all Parfocal with only the 2x Powermate requiring a slight focus shift. See
https://www.televue.com/engine/TV3b_...id=53&Tab=_app