2nd monitor on Late 2015 5k iMac

I did a search on 2nd monitors in this forum and the threads all appeared to be years old. So I'm asking (probably again for many) now. I have late 2015 iMac with 27" 5K monitor and Thunderbolt 2 ports. If I want to add a 2nd 27" monitor -

1. Am I limited to a 4k monitor due to the max bandwidth of Thunderbolt 2?

2. If I am a photographer using Lightroom/Photoshop a lot, will I see much of a difference between the 5k and 4k monitor?

3. If I add a 2nd monitor on the 2nd Thunderbolt port, does that impact the performance of my RAID array I have on my 1st Thunderbolt port and vice versa?

Thanks for any help.