You really need to get the car on a dyno to optimize your timing adjustments. Seat-of-the-pants feel can be pretty misleading when timing is concerned....lots and lots of 4.6 owners have discovered that the setting that felt best to them was not producing the most horsepower when the dyno sheets were examined. This is probably the rule, not the exception.
That said, I installed mine at 13 degrees (huge difference over the stock setting), then bumped it up to 15. After a week or so I moved it back to 13. It ran fine on both settings but I really didn't notice any difference between 13 and 15. I need to get my car on a dyno, too, before I do any further adjustments.