Why do so many of you on this forum want to reengineer the vehicle? The professional engineers who designed our cars made the compromises they did for reasons that may not be obvious to us.
If you all are so smart go design your own vehicle get it certified all over the world and sell it and become a millionaire. Think Bricklin, Delorean, Tesla, Shelby etc.
Flawed on so many levels.
First, engineers don't make all the decisions. If they did, whatever they made would be too expensive every time. Upper management, marketing, government regulations, etc. all have input that engineers have to work with.
Second, compromises are made to best fit most consumers. I make my own "compromises" to best fit me. For example, I sacrificed economy for performance. I'm pretty happy with the results and would never go back.
Third, there's nothing wrong with a little experimentation. Sometimes you hit, sometimes you miss. That's how we learn.
Fourth, no need to design a whole entire car just to implement a few features, when there is already a car that's 98% of what you want. That's actually the opposite of smart.
With that said, what the OP asks is pretty much impossible until we get the right people in our community. You need to be able to hack into the IMA ECU or perhaps be able to have a "piggyback" module modify data on the IMA CANBUS to trick the car into auto stopping sooner. This person that makes this happen would be doing this out of the kindness in their heart, because hardly anyone will pay enough money to make this effort worthwhile...
I suspect the main reason the autostop is set at the speed it's at is because they want you to regenerate, as Koala has stated.
While the merits of modifying the autostop speed are debatable, there are quite a few other reasons to hack into the IMA ECU. Final gear ratio changes and increasing rev limits have some minor issues with the IMA that could be fixed with some simple software tweeks. IMA assist drops off after 6500 RPM. IMA quits working altogether after 7200 RPM. Final gear changes screw with regeneration. All can be worked with as is, but it would be nice to have to the ability to make those changes...