A higher dose is no more effective than a lower dose. One mg and a half a mg basically reduce estrogen the same amount, except for a few percentage points (not relevant).
From my first book:
" In one study, both .5mg and 1mg doses of Arimidex were shown to decrease estrogen by roughly 50%. The 1mg/day dose also increased testosterone levels by 58% (1). "
1. J Clin Endocrinol Metab 2000 Jul;85(7):2370-7, "Estrogen Suppression in Males"