Auto update stats turned off for a user database on SQL Server While trying to improve the performance of few queries, we found via execution plan that there were lot of index / clustered index seeks. 

Therefore:

First thing we did, was to check our Re-indexing and update stats job which runs weekly for this user DB ( Around 400 GB in size and is used 24*5). The job was running fine.

Later when we ran SP_Blitz, we came to know that auto-update-stats is disabled for this user DB. We expected this to be a possible cause and change it from false to TRUE (Auto update stats)

Also, per SP_blitz there are user-created statistics for this DB. When ran the query to check how many, we saw around 7K user stats out there.

So my questions would be 


setting the Auto update stats to TRUE would require a reboot or once changed I need to track the performance 
Should we consider dropping those user created stats or manually look into them one by one?


How should we proceed on this, please suggest, thanks!
You have two questions:

1. Do you have to restart after setting auto update stats on? No.

2. Should you consider dropping the user created stats? If your statistics updates jobs (typically done with a maintenance plan or with Ola Hallengren's maintenance scripts) are taking longer than your maintenance window allows, then yes. SQL Server has to read the entire index (or table) to update each statistic, so you can end up with really long stats update jobs. If your maintenance window isn't a problem for you, then you can leave 'em as is.