Switching to JSON instead of text has several benefits:
1) The data will be verified to confirm to actual JSON specs. Your server-side app might already be doing that, but type safety is always nice.
2) You have access to several JSON functions, such as getters of properties at arbitrary paths, conversion to records, and more:
http://www.postgresql.org/docs/9.3/static/functions-json.html http://clarkdave.net/2013/06/what-can-you-do-with-postgresql-and-json/
To do setters, I suggest a simple plv8 function that takes 3 inputs (json object, path-to-property, updated-value) and returns the updated json object back. That can be used directly in update statements.
3) You can actually index certain properties of the JSON already for performance:
How to create index on json field in Postgres 9.3
4) JSON 9.4 will introduce binary storage, along with index engine improvements will drastically increase speed:
http://obartunov.livejournal.com/177247.html
Here is a nice talk on the state of JSON in 9.3:
http://www.slideshare.net/amdunstan/93json-26647827
Here is a performance report of an earlier version (using the original hstore2 work) showing some performance relative to mongodb - this is by the jsonb author:
http://obartunov.livejournal.com/175235.html
and an updated benchmark from someone else: