I contacted Garmin about this issue and they requested me to fill out the ideas form!
http://www8.garmin.com/contactUs/ideas/
XT Shutdown message on lost power
Re: XT Shutdown message on lost power
I find this glacier-slow shut off process a nuisance as well. I also sent Garmin an email which will do absolutely nothing...but it made me feel better.
BMW R1250GS - Zumo XT / Cardo / iPhone.