60 % survive rate sounds pretty good. On a good week I find myself getting 6/7 of successful defenses, but bad weeks are about 50/50. It will probably not get much better than that, as it is a bit of luck of the draw on who attacks you. If it holds up in wars, than you can probably hold off on adjusting. Top teams will always be top teams regardless of money/time spent. They will be the first ones to crack any new defensive updates that show up in future updates.