In the slow paced MMO that FFXIV is, increasing server-side polling will have a very minimal impact. The only real change it would have are the split second decisions for players with solid connections to hit off faster. Such as using hallowed ground whilst the animation of an incoming attack would already have started. And still have the hallowed ground cast hit off prior to the incoming damage.

However, for those with crappy connections, those who currently fail to get out of the AoE ground effects and such, absolutely nothing is going to change. Better yet, by ensuring that players with a good connection get to play even more solid increasing server-side polling may even bite you back. As in comparison you'll end up playing even worse. After all, increased server-side polling is going to do absolutely nothing about delay and packet loss on the route between your PC and the FFXIV servers.