While the gripper seat may help some, arm fatigue is usually caused by riding position. The best position is standing, bent at the waiste, with your head far enough forward so you can see the front number plate. In this position, your arms are more of a 90 degree angle to the bars, so your not actually hanging on to the bike with your arms, your just working the controls. This is the only way that riders are able to ride those multi-day races for long hours each day, as you would have to have Popeye arms to ride these events holding on with your arms.
With the addition of the gripper seat, work with your riding position. You will be surprised at the difference in body fatigue.
Rick