How accurate is XFoil / XFLR5 at 'kinda high' Reynolds numbers?
Before version 6.9 (30 Nov 2001), XFOIL had some problems at Re > 3 million, but that's now been fixed.  Its documentation says:


  The BL equations are normally discretized with two-point central
  differencing (i.e. the Trapezoidal Scheme), which is second-order
  accurate, but only marginally stable.  In particular, it has problems
  with the relatively stiff shape parameter and lag equations at
  transition, where at high Reynolds number the shape parameter must
  change very rapidly.  Oscillations and overshoots in the shape parameter
  will occur with the Trapezoidal Scheme if the grid cannot resolve this
  rapid change.  To avoid this nasty behavior, upwinding must be introduced,
  resulting in the Backward Euler Scheme, which is very stable, but has
  only first-order accuracy.  Previous versions of XFOIL allowed a specific
  constant amount of upwinding to be user-specified.  Currently,
  XFOIL automatically introduces upwinding into the equations only
  in regions of rapid change (typically transition).  This ensures
  that the overall scheme is stable and as accurate as possible.