I used an acrylic-based artist gesso on my 50-pounder last year. I'd been wanting to try it for several years, but was too uncertain about it to try it on a client's canoe.
It worked pretty well, and I would use it again. Some observations:
1. I was able to apply 3 coats in 1 day, allowing each coat to dry between reapplications.
2. I let the last coat dry for 2 days before sanding and painting, but could have painted after a single day's drying. This was much quicker than a traditional filler.
3. I didn't weigh the gesso, but I didn't seem to save any weight over traditional filler.
4. It might have been because I was in a rush to get the canoe in the water and didn't spend my usual amount of time in sanding the filler, but it seemed a bit more difficult to get an even fill with gesso than with traditional filler.
5. Being acrylic-based, cleanup of tools was easy, and oil-based fumes were not a concern.
6. The paint finish coats adhered very well.
7. I've not used the canoe extensively, but have had no problems with the canvas being watertight.
All in all, I found gesso to be an acceptable filler, particularly if I'm not inclined to wait the usual ~3 weeks for traditional filler to dry.