I think overtaking cars purely on pace is excessively difficult. You can easily observe how this is a problem during wet races, or at the start of the race if a good car is heavy, it will create a big gap and cars behind can't do anything.
It definitely takes away an extra element from the game.
I bet every good manager has found himself stuck behind slow cars. You might say this is part of the strategy, that you should have known better, but I disagree, this is just using the current system to one's advantage.
Imagine the possibility of pushing less to save tyres and attack later, or making an aggressive strategy knowing that softer compounds will make a difference in a train or even the opposite, using a hard compound not to overcut but to overtake later in the stint.
These examples in theory should already be true but I believe that the current overtaking system is negligible in the overall of a race.
In conclusion, make deserving overtakes happen.