Ben,
Depending on whose ad you read, ultra cam and 450+ premium both have between 125 and 150# breaking strength per strand. I've not considered that something that needs to be nailed down, because strings don't fail by being jerked apart end-to-end. I think poor construction, unequal tension between strands, and abrasion are what fails a string. Ultra cam is as narrow as any material I've seen and 450+ approaches B50 in diameter. But both are lighter than B50. I usually find a well made 14 strand B50 string with silencers will weigh in at 175-200 grains. I can do the same thing with 6 strands of ultra cam with 6-8 strands of B50 padding in the loops and come in below 100 grains consistently. 8 strands of 450+ for the same bow would weigh about 110-115 gr. In my experience, the low-stretch materials take less silencing, and that also contributes to the mass difference. It's almost never a part of these discussions but I am convinced that cutting string mass in half simply has to be right there with low stretch as a significant difference in energy transfer to the shaft.
I have not experimented with other string materials at all. I have a lifetime supply of B50 in a lot of colors, and early on I settled on Ultra Cam for the skinniest string, and 450+ because it's easier to make the nocks fit better. When I run out of those materials, I'll try some other stuff!