Here's some background information. The T level ranges for adults according to healthline.com are:
Male 270-1070 ng/dl (up to 1200 as adolescent)
Female 15-70 ng/dl (up to 75 in adolescence)
I've read somewhere that some top female athletes have T levels in the 90s, but I can't find a source right now.
The IOC rules allow intersex athletes to have a T level of 10 nm/L, or roughly 288 ng/dl. That's FOUR TIMES HIGHER than the normal range for women.
The reason there is no clinical proof of T levels conveying a performance boost in intersex athletes is that intersex is so rare that it would be impossible to put together a valid clinical study. Even within intersex, there are other confounding variables that could invalidate a study. Plus, how could you find enough intersex athletes AND convince them to participate in a study that would most likely result in banning them from sport?
Further, there have been several comments about Semenya having to take female hormones. The ruling says only that intersex must take hormone therapy. I'm pretty sure that hormone therapy in this case is a testosterone inhibitor, not just more female hormones.