I've decided to run the manual update collection since this update (7.8) was the only reason I didn't already move to another carrier to get a windows 8 phone.
So.. for the arrive on sprint it looks like there are 7 update steps.
The most updated build from HTC on sprint is 7720, Here's the update chain
7720 -> 7740
7740 -> 8107
8107 -> 8112
8112 -> 8773 (tango)
8773 -> 8779
8779 -> 8783
8783 -> 8858 (Windows Phone 7.8)
I've just completed tango and am downloading the next collection of updates now:
8779 ->
OS:
http://download.windowsupdate.com/m..._b9598a098a3a4e902d043cef778a79ff3e6b0869.cab
Languages:
http://download.windowsupdate.com/m..._4d87f62a11c7898a516df7563a9333e1082fad57.cab
8783 ->
http://download.windowsupdate.com/m..._8a50b92b21951c7baf9f3600cea8c77e0918021f.cab
8858->
OS:
http://download.windowsupdate.com/m..._d6fb2a271eed6a5d1187b17aba60806753d0ffce.cab
http://download.windowsupdate.com/m..._39b9cc884c90d83c0df50ed5623166e9257d1162.cab
Languages:
http://download.windowsupdate.com/m..._653898f93f54d951992766d317db1b846ca71e36.cab
only 3 Steps to Windows 7.8 worked for me!