I dealt with rubber and urethane for 35+ years so, I can help with some of the questions. I am still trying to get more info in regards to the existing tooling before I buy it.
Hardness of rubber is measured with a spring loaded pin. How deep it goes determines the hardness. The deeper the pin goes, the softer the material. There are at least 4 ranges of hardness and is measured with a durometer. Shore has A, B, C and D scales. Shore A and D are by far the most common ranges. They overlap and there is a reason for that. At the outer ends of the scale, the measurement becomes less accurate. So the optimum accuracy is in the middle of the range. You can google it and see what I am talking about. So a 70 Shore A rubber is a very common hardness and what most O-rings are.
But, when you squeeze and bend rubber, that is NOT the hardness you feel but rather Modulus. Modulus is very commonly mistaken for hardness. "An elastic modulus is a quantity that measures an object or substance's resistance to being deformed elastically when a stress is applied to it". Not my words but that is what it is.
A good example is that urethane has a much higher modulus than would common rubber. That is the property you are after for suspension bushings. Compare a 80 Shore A urethane to 80 Shore A rubber and you will find the rubber bends much more easily. Yes the harder the material the somewhat stiffer the material feels, but modulus is more important here.
That being said, I could see using a range of 85 Shore A to 95 Shore A urethane. Any softer than 85A, you aren't really gaining much over OEM rubber. 95A is hard as a rock and most would call a plastic. Plastic BTW, is usually measured in the Shore D range. 95A is equal to 55D as a reference.
I am not however an expert in suspension systems and I could see experimenting with hardnesses. Yes, I think you could offer a mild upgrade and then the higher performance bushings. Each with its own pros and cons. That would not be an issue in production to do so.