I know what you mean about having too much time to think! I also looked at the $8k diff back when I finalized. But don't forget tax on top of it. But the wait for them to build more 60s is maddening!
I'm paying $700/month for a base Ford Fiesta plus gas while waiting. Every now and then (usually when I need to buy gas, or have to hear the little tiny engine struggle to make it to highway speed on an onramp) I realize that I'd be in a MS already if I went 85.
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Anyways, in case you might be wondering, I just checked. Not possible to change from 60 to 85 after finalizing. Before flaming me, I expected and accept this. I just had to check