"1.) Going to make it out of M90 instead of M50, as that is what the Thru-Hiker kit contains.
2.) Going to make it wider than Jamie's, probably around 55 inches wide.
3.) Going to sew in the footbox."
Sounds like you'll be making my quilt, give or take.
Made with Jamie's instructions, 54" width (very generous for my scrawny self, but good for my propensity to toss and turn), sewn-in footbox, made with .9 oz ripstop from OWFInc (cheaper than the M90 at TH).
If you're making a summer quilt, you may rethink buying the TH kit as it contains far too much down, noseeum you don't need, and costs more than just buying the fabric from OWF and the down from TH separately. You don't save much though, and if you sell the extras you don't use from TH's kit, you'll probably end up ahead.
There's a thread on my quilt in MYOG section too if you care to dig for it, it was within the last 2 months or so. Might give you some ideas on the sewn-in footbox and such. Feel free to PM if you have questions, and I'll do my best to answer them.