we're using the latest GWT version in master UI now .
Effectively, all Engine (Java Maven) modules  are now Java 8 source & target compilant.
 except backend/manager/modules/extensions-api-root
There are no changes to existing build & development process:
- debugging via Java IDE (aka Classic Dev Mode) remains the default debug method
- debugging via browser (aka Super Dev Mode) can be enabled via DEV_BUILD_GWT_SUPER_DEV_MODE flag 
 example: $ make gwt-debug DEBUG_MODULE=webadmin DEV_BUILD_GWT_SUPER_DEV_MODE=1
Note that in GWT 2.8 the Super Dev Mode is the new default debug method (with Classic Dev Mode being deprecated).
If you encounter any issues related to GWT compilation or debugging, let me know.