In my experience it restarts each time then it comes to GUI mode. So, it loses "renice" value and returns to normal during GUI. But anyway, "renice -1" is actually very slight difference with normal priority. It is very difficult to note about it.
method call sender=:1.214 -> dest=com.nokia.modest serial=497 path=/com/nokia/modest; interface=com.nokia.modest; member=top_application
method call sender=:1.214 -> dest=org.freedesktop.DBus serial=490 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=StartServiceByName string "com.nokia.modest" uint32 0
Yes, but if it process is nice then the device won't consider its processor demands when considering whether to switch frequencies and voltages.
It looks like the way to make modest nice is using dbus-scripts.
This dbus signal is sent when you open the mail GUI: