Hey Sam,
I've been messin' around with that a bunch myself lately. I could never claim to have a senko replica as nice as Yamamoto's, but I'm happy with the action on mine. I use Calhoun's "hand pour" plastic, which is about equal to Lure Craft's 536 formula. I add 1/2 cup of softener and 3 ounces of heat stabilizer per gallon of plastic.
As for salt content, I like to mix 1/4 teaspoon of popcorn salt per ounce of plastic, mixing like crazy to keep it suspended. I'm sure you know that extra salt makes the bait much tougher and much more difficult to bring the plastic to a "runny" state. The stabilizer and softener make up for that. I set aside a separate 5-gallon jug of plastic just for senkos. There's no need for stabilizer if you're not pouring with salt in the mixture. Some guys will say the softener is not needed also.
I don't know how Yamamoto does it, but as you know......you can actually see and feel tons of salt in their senkos (I was told their mixture is 60% salt!!). I can't see the salt in mine like Yamamoto's, but when I test them in the water, the action is the same. Mine might be just a hair softer, but they are more durable. In fact, I usually catch more fish on mine because of added scents.
This is by no means a push for my product. I just believe that anyone who pours plastics should pour their own senko replicas. With a little effort, you can make a really nice bait at a fraction of the price.
Chris