According to UESP, there is a cap on armor rating effectiveness. I was wondering if anyone knows or can direct me to a website or something that knows at what level of improvement each armor reaches its effective level cap?

It doesn't work that way. Your overall armor rating is what counts: it caps at 567. Technically it's 667, but each armor piece has a hidden rating adding up to 100 if you're wearing a full set (head, body, hands, legs). This value equals 80% damage reduction, the highest you can get.

Edit: wait, you're asking about armor sets. I think I saw it somewhere on this forum... I'll try to find it.