Från 2+2 FAQ:
What are attainable stats at the different levels?
NOTE: The below talks about the "old" party structure. Conclusive sorts of findings with respect to the "new" party structure haven't been come up with yet.
In this thread, one poster comments on many things, including attainable stats for the various buyins. More than that, there is quite a bit of posting later in that thread about what are attainable and realistic ROI’s for the higher stakes Party SNGs. A summary for a "good player" the following ROis are good, while 4 tabling:
11s - 25% ROI
20s - 20% ROI
33s - 16% ROI
55s - 13% ROI
109s - 10% ROI
215s - 7% ROI
Higher ROIs are attainable at every one of these levels, HOWEVER if you can achieve the above ROI's you are capable of making a good living at poker, especially if you can do so playing 8 at once.
Anyone who can acheive the above ROI's, while playing many tables at once, is a "good" player. They might not be great and may still have plenty of weaknesses, but they are definitely very good at what they do.
In short, if your ROI is higher than the above numbers, you are doing something right and should be relatively satisfied with your play. You may still have plenty of room to improve but you are definitely on the right track.