This is a really interesting thread, because if the topic were "playing *faster* than your level", the received wisdom is a little different - playing above one's comfortable speed means scrappy playing, and that means that scrappy finger placement is what becomes learned. The way to increase speed is to play about 90% of one's top speed and make that playing more relaxed, so that the technique is optimised. Once that happens, speed can be increased.
As a teacher, I find that there is a huge amount in "pieces that aren't too hard" that encourage musicality, phrasing, expression, tone production - all the things that tend to get pushed to one side when one plays something a bit too tough.
My approach isn't to strive to play 3 harder pieces, it's to play 30 easier pieces more comfortably. At the end, the audience will prefer your music and you'll have ten times as much of it and a set of skills that are much more well-equipped to help you tackle tough-stuff just a bit later...
Unless you want to be the next John Williams, it's probably better to broaden your enjoyment and repertoire instead of torturing yourself - we can't all play really hard pieces, any more than athletes can all jump this world-record high-jump.... Unless you want to be the next John Williams, you should explore music, rather than technique, or you will never be satisfied