Last night I merged DM-7010, which enables optimisation for builds by default. That means that you can get rid of the opt=3 in your SCONSFLAGS (which can break other products built by scons).
I also added support for the -Og optimisation flag for g++, which optimises “debugging experience”. The gcc manual says “It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience.”. You can activate this by adding opt=g to your scons command-line.