Excess velocity and excess powder charge can reduce accuracy and increase wear. Your source mentions a lot of muzzle flash from the 17 pounder and speculates that could be from the powder charge not burning fully. Another possible reason for that is that there was gas leaking around the shell while it was still in the barrel. This could be a consequence of greater throat erosion. Increased velocity and increased barrel length do not necessarily give greater accuracy. You may want to check out r/reloading.