Quite possibly, but it depends at what stage APR code out the post cat lambda monitoring.
In terms of 'safe', yes it's fine to add a downpipe without a retune. The wastegate control map will adapt and smooth out any torque spikes, which you will definitely get initially because of the huge reduction in back pressure.
An APR downpipe bumped the torque up 30lbft and power up by 20hp on my ED30 without touching the map. Obviously you'll get a lot more if you take advantage of the downpipe and dial in more timing and boost.
Consider an intake if you want harder pull past 5500rpm, but don't expect much more than 15hp gain from that.