You are on the 401 not the 778? The thickest I have sewn with twin needles are flat felled seams, hems, and edge stitching on cotton. As far as I can remember I have mostly used sew-all weight Gütermann (polyester) and mercerised cotton. I never got the feeling it was noticeably heavier for the machine, but I guess the needle punching will be double the work. These machines can take a bit of struggle, but it's usually seam bumps and not the long stretches of continuous stitching. I guess you would have to pull out something like a Pfaff 360 or other model with sturdy build and extra strong motor to improve on the 401. If your other 401 does the job easier, at least you know the first needs some attention. I have pushed my Bernina 730 and Elna Supermatic to their limits, and no damage to them. I don't know about the 401, but the mentioned zigzaggers sort of just stop stitching when it's too much, and it's hard to overload them in the long run.