IMO, not worth it - been there, done that.
The sound quality and sound-shaping capability of the stock HU is rather limited. You would need an amp with speaker-level input (not a big deal) and then tie back into the factory harness. By the time you get done screwing around with this, you'd be better off installing an aftermarket HU. Most have enough power to make the stock speakers sound better.
FWIW, I retained my stock HU for a long time for the very reasons you list. Ultimately, after adding aftermarket component speakers, an amp and a powered subwoofer, I ditched the stock HU and am now finally happy with the system.
Sadly, the OEM sound system in most Subies is pretty bad. The factory system in my wife's '09 Volvo S40 absolutely blows away the "premium" factory system in my '07 FXT Ltd. (there is/was seriously no comparison).