" I can't find any stories about trans women sexually assaulting cis inmates in women's prison but I'd be open to any studies that give different numbers"
No one knows how many trans prisoners there are, although the Home Secretary has asked for the numbers to be collected. I have seen an estimate of 80-100 in the uk.
As I understand it, as in the UK numbers have largely been limited to trans women with GRC, who may or may not have had surgery but are certainly reasonably likely to have done so. If the definition of transwomen permitted to enter a women's prison is broadened, it is unknown what the effect will be on any stats.
Is the 54% from the US, given the lack of UK data?