  To meet critical current density, J$_c$, targets for the Future Circular
Collider (FCC), the planned replacement for the Large Hadron Collider (LHC),
the high field performance of Nb$_3$Sn must be improved, but champion J$_c$
values have remained static for the last 10 years. Making the A15 phase
stoichiometric and enhancing the upper critical field H$_{c2}$ by Ti or Ta
dopants are the standard strategies for enhancing high field performance but
detailed recent studies show that even the best modern wires have broad
composition ranges. To assess whether further improvement might be possible, we
employed EXAFS to determine the lattice site location of dopants in modern
high-performance Nb$_3$Sn strands with J$_c$ values amongst the best so far
achieved. Although Ti and Ta primarily occupy the Nb sites in the A15
structure, we also find significant Ta occupancy on the Sn site. These findings
indicate that the best performing Ti-doped stand is strongly sub-stoichiometric
in Sn and that antisite disorder likely explains its high average H$_{c2}$
behavior. These new results suggest an important role for dopant and antisite
disorder in minimizing superconducting property distributions and maximizing
high field J$_c$ properties.
