Will a covering index save a table hit if only the commoly-used length of a `text` field is indexed?
It depends.


Prefix indexing INDEX baz(800) is rarely useful.
MyISAM and InnoDB work dramatically differently in the area you are in.
In InnoDB, if id is the PRIMARY KEY, it is almost useless to have an index starting with id.  This is because the PK is clustered with the data.  For MyISAM, the PK is a separate BTree, so the analysis is quite different.
If you are using InnoDB, which version, etc, is the table?  Antelope, ... COMPRESSED, ...?
Adding extra indexes adds more stuff to cache, thereby possibly slowing down other queries by bumping things out of cache.  Since your 'optimization', if it works, is not likely to save more than one disk hit, you may not actually be speeding up the system any.