Specific different in fruit preserves vs. fruit pie filling? I've made preserves a lot lately.

Cornstarch is used to thicken pies! Pectin is for jelling fruit for jams, jellies, and preserves! Hope this clears up understanding.

One good way to thicken pie filling without cornstarch is to grate a tart apple and add it to the pie filling. The added pectin from the apple helps achieve a nice set without affecting the flavor of the pie or being gummy/cloudy.

Are the fruit to sugar ratios around the same in pie vs. preserves?

No, not even close. Conventionally made preserves have a much higher sugar/fruit ratio than pie fillings.

What about desired texture?

For most of us (I think), what makes a fruit pie a fruit pie is discernible pieces of fruit that you can see and taste, as opposed to the more-or-less homogenous mixture that is a preserve.

You raise a good point, though. One way to reduce the need for starch-based thickeners in a pie is to cook part of the fruit with sugar to a jam-like consistence before baking, while leaving the rest raw. This does two things to the filling: reduces the total amount of liquid and increases the proportion of pectin. In effect, you're making a sort of jam with part of the fruit while leaving the rest raw. This technique is most successful with high-pectin fruits like tart apples, sour cherries and plums, less so with low-pectin fruits like blueberries and peaches.