I had my 7 on the 1.5", didnt hold worth a damn. Ball was destroyed the first year, had to buy a new one.
My 10 used to be on the 2.5", in rough water, it slowly moves down. I didnt trust it, so i replaced it with more of a fixed mount. Didnt want it moving to hit the windshield or dash. Maybe i was just unlucky with mine.