My observation is the 722K and 222K does, indeed, have a slight, but noticeable better PQ. In fact, the 222 PQ also looks even more noticeably better than the 722.
Best PQ's in my household:
1. Tivo Series 3
2. 222, 222K, 722K
3. 722
The differences between 1-2 are VERY slight, but those 2 are noticeably superior to the 722. However, the 722 still has great PQ, and you would not notice unless you had other boxes to make a comparison.
Yes, the quality of the hardware inside (chipset, etc.) and the bandwidth in and out of the HDD will make a difference. There is a noticeable difference between the Tivo Series 3 / HDXL and the Tivo HD, for example. In other words, the THX certification is something that can be appreciated in the PQ.