Headphone performance is only possible with headphones. Although, you can get killer staging and imagining with the tuning features of this head unit. The DSP in the P99RS is "limited" compared to standalone DSP's, but it has more than enough power to make a pretty amazing system. If you know how to use it, you can make it sound pretty awesome. Most people never fully learn to optimize it before going to a standalone DSP because they "need more processing."
Of course, you are correct - Having moved away from a bootful of Audio Control Gadgets it would be interesting to imagine and see just what a full blown DSP could do over and beyond where the P99RS is at - and I agree, when max'ed out, this headunit can do some quite amazing things.. for me, it has given me a way better system than my previous EMMA competing system and done so with a lot less equipment..