Nokuthula Ngwenyama was one of my favorite viola soloists when I started playing the viola. Some time around 2010 she seemed to disappear from concert stages, which seemed odd because she was only in her early 30s, and not too long ago I was just wondering what happened to her when one of her pieces ("Primal Message" below) was posted on YouTube. It turns out she cut back her performing schedule to focus more on composing, and seems to have reinvented herself as a rare modern-day virtuoso composer-performer.
Here she performs her own "Sonoran Storm" (composed 2016) for unaccompanied viola. This piece is inspired by her home state of Arizona and the impression I get of it is Bach-meets-American-West.
(Just a side note for those who are learning viola: this video is great for showing how viola bowing technique differs from violin! She plays the fast staccato lower on the bow than violinists would, which allows a firmer grip on the strings; and you also see a more pronated wrist to dig into the string.)
And here is her "Primal Message" for viola quintet (composed 2018), performed by the Dover String Quartet plus Ngwenyama.
I don't think so. I hear nothing below the normal viola range and nothing that sounds different from what I would expect a good viola to sound like. There is a big difference in sound between a 5-string violin and a viola -- about the same as the difference between a half-size and full-size violin.