It is shown that a significant increase in the consistency of charge exchange spectroscopy and x-ray spectroscopy deduction of central ion temperature and plasma rotation is obtained when allowance is made for the following: i. The charge exchange cross section dependence on the impact energy of the neutral beam atoms in the frame-of-reference of the C+6 nuclei. In this paper we have experimentally verified the predicted cross section effects on ion temperature and plasma rotation measurements by comparing to passive x-ray spectroscopy. The effect increases with lighter species. ii. The precise position of the radiating ion shell in line-of-sight averaged measurements using x-ray spectroscopy. For high temperature plasmas with broad emission profiles the uncertainty in central ion temperature is reduced from up to 60% to the order of 20%.