I have heard a great deal about how good he is with the ball at his feet........but is he as good as Joe as a keeper ?
To be honest I'm not sure it matters that much.
The way to think about it in my opinion is like this - ter Stegen in goal would allow us to push the defence even further forward. That compresses the pitch and makes our pressing more effective, which means snuffing out a lot of attacks before they develop. It also means the opposition forwards are further from the goal and it means we'll be much better at holding onto possession and keeping the ball is the best form of defence. Compressing the pitch more also means shorter distances for our players to run when they press, which means they can maintain a higher intensity for longer.
So even if his being a slightly worse shot stopper costs us a goal or 2 that Joe would have saved, the tactical changes it would allow us to make would likely save many more.
On top of that, you've got his contribution to the attack. If we had him in between the sticks, he would be a genuine passing option when playing it out from the back which gives you a man overlap somewhere and that free player will create goals when the ball is worked to him.
The "good with his feet" thing really cannot be overstated in terms of importance.