Hope I'm not overdoing my experience with a new mid-drive kit, but seems as though it would be applicable to commuters. Just completed a build with a Revel Propulsion mid-drive. The kit added 12 pounds (10 for the motor and ancillary parts and 4 for the battery, while replacing slightly more than two pounds of crank/BB). There are several nice things about the system beside the fact that it closely resembles Yamaha and Bosch OEM's in performance. These include, but are not limited to, switching from 20 mph assist to 28 just by pushing a button on the display, motor nesting very close to the down tube, not hanging down like a cow's udder, very good instruction manual (on website) and nine utube videos to facilitate assembly, ease of mounting/ removing battery, connections are waterproof and no spaghetti (preponderance of wires that must be dealt with).

Very nice! I built my lightweight e-commuter about 6 years ago. Makes in fun to get to work! I'll say though, ebikes really chew up knobby mountain bike tires in no time at all. You'll probably want something slicker and tougher for the rear (I've been using Serfias drifters).

C58; I use this for MTB, although tried 28 mph on the street to see how the system preformed. Posted the picture to show the motor/downtube interface. Weight of bike is slightly less than 36 pounds, nowhere near you're 25 pounders.

36 is quite doable for a mountain bike. Most of them seem to be 30lbs out of the box anyway.
(mine is 25lb w/o battery, 29 with battery - so this isn't that far off...

The steel frame is six+ pounds; could have reduced it by three pounds with my Scandium Niner One9. However this bike is really fun off road, so I can live with the extra weight.