Personally, I've never used the 24, but I can say that if you remove the mulching plug and side chute cover, they'll cut through the longest and toughest of grass.
Pau,
Yes, I'm rural and I took the side shoots of each of my mowers that have them. I'm pretty fastidious about ensuring no stones etc on my lawn so this was done with great consideration as I have very large glass windows on my house.
But yes you're right it makes a big difference - I took the side shoot flaps off the 550 Pro's I briefly tested and when I went through quite overgrown kikuyu with it the best way to describe the grass ejection was imagine a car had just driven at high speed through a puddle of water sending it over a nearby pedestrian - except then that the water is instead grass clippings.
I actually have to dial that back a tad as it was too strong - and so it had 4 blades on it, when I get it going again I will try with just 2 - if thats still throws too hard I might take some of the fluting out of the blades by flattening very carefully in a vice.
I find if you use the flap they do clog up more but also it leaves thick lines of very thick clippings....without the flap it tends to scatter them evenly so they're less noticeable.
