IMO, the ASIO perf is most probably related to the particular vendor's implementation. On my current setup, using an AMD X2 3600 O/C'd to 2.4Ghz, running the RME Digi96/8 PST, when playing back music with MC12 (High APE), she goes about 1-2% of the CPU. I run no DSP.
When I had my old Shuttle running it (RME ASIO w/MC12) with an O/C'd Celeron, she was running <7% CPU.
As to discernible audio diff, well, that will depend on the articulation and resolution of your system and the chain that supports it. I can tell you that on my system, I can hear a diff between ASIO and other output, and that is using three different speaker combos (Von Schweikert, Monitor Audio, Hammer Dynamics) with the same pre-amp and amp setup, using an external DAC or the DAC on the RME. Even my wife can hear a diff, but in that case, she has a better ear than me anyway.
ASIO supports > 2 channels.