I just went through the whole shebang starting with a 2.4 in 2018.
4-5.5 was nice, I noticed a big difference with rinsing speed and concrete went a little faster, but I was getting frustrated with striping. SH pull from the injector was about perfect with 5.5.
5-8 jump was a real shock. I can now get 3 story peaks from the ground using my 1’ wand and shooter tip. Surface cleaning is night and day speed wise, I almost walk normally behind my 19”. Rinsing speed is ridiculous.
Drawbacks, my 2.1 injector doesn’t draw as much as I’d prefer so it usually takes two applications, especially on gutters. I’d use a 1.8 but as I use a remote, trying to rinse with it still on cuts down on GPM. Also the sheer volume of water can pile up quick when rinsing off concrete so low spots need to be dealt with. I personally have to watch my 65 gallon leg tank so it doesn’t run dry when people’s spigots are lower flow.
My biggest regret is not spending the extra $500 to get the 8/3500 instead of the 5.5. I could have avoided so many problems.