Which tables are you trying to query? I believe that this history tables (prefixed by hist_) have 183 days retention by default. This can be changed with the tabadmin option wgserver.audit_history_expiration_days. tabadmin configuration options . The only impact is that your Postgres database is just a bit larger, but if you have sufficient disk space, then it should be fine.
If you are talking about amending one of the existing Tableau Server Admin views, then this isn't directly possible. However, it is possible to get a copy of the workbooks used to build these views which you can then connect to, amend and publish elsewhere.
You will need to have set up access to your postgres database. Please have a read of the following
What the impact of extending to include more than 90 days will be dependent on your set up, the volume of users (and therefore amount of data being collected) and which views you actually change. Note though, that as per the first article, there is also a limit as to how much data Tableau will store by default (183 days). This is a figure that can also be amended, but again what the impact will be I could not say.
Performance data on how long dashboards take to load for users is stored only in http_requests. It is maintained only as long as seven days, and the extra records are removed when you run tabadmin cleanup. There is no setting to preserve that data any longer. To do so, you will need to find a solution to incrementally pull new records out into a separate location. An easy way to do this is by using an incremental extract in Tableau, and just incrementing it on the "id" field. But make sure you don't do any full refreshes, as doing so will wipe out everything accumulated so far. Since certain issues may require you to take a full refresh every now and again, it's best to back up what you've accumulated so far every month or quarter or so, and if need be, use something like Prep to union all the data together.