But the purpose of suburi is to develop correct form. A modest amount of extra weight will help with that goal, a large amount of extra weight will impede it, IMO.
I dunno. I do suburi as one method of training and I have 5 weapons ranging from a lightweight hickory-heart bokuto to a 17-pound ebony one. While some exactitude in replicating a sword-cut is of course desirable, I think for Aikido it's more a form of tanren. I teaches you how to use the body and it trains/conditions the body by using the same body-strengthening principles that are used in internal strength, use of the hara, developing the ki and developing the kokyu.