The 8.1.x software TivoHD has most of the new features found in 8.3.x.
As far as I can tell, it is only missing the indexing changes. It still indexes and searches every channel in your OTA and cable lineups, regardless of whether or not you actually receive them. It hides the results for channels that you have removed but it still searches them.
With 8.3.x, only the data for the channels you have checked is indexed and searched. This dramatically improves responsiveness with the guide and searches.