Pure speculation:

If I work my @$$ off really hard on my own, on very flat grounds, assuming no wind, I can manage to hold maybe 30 km/h (imagining a road with no obstacle) for about an hour.

If I work the same way but instead of being alone I'm holding to my boyfriend's wheel for my dear life, we achieve about 37-38 km/h on the same flat road with no obstacles. (He's working hard too, but not TERRIBLY hard. I, on the other hand, will need to eat a whole bear - rare, please - at the end of that hour, with a cold beer if possible! .) We've done that a couple of times.

So the 20-30% estimate is about right from my experience.

I read somewhere that there was some - minimal - positive effect for the person pulling at the front, something like 5-10%.

Now if it's a club ride, even if we're on a paceline, there's lots of variables to consider: who's there, how experienced they are, what terrain we're on, how chatty we are, how hard was the race the week before, etc. We rarely go more than 2-5 km/h faster than I would go on my own.