With high back boosters their head can sometimes go over the top. You want to look at the position of the shoulder belt in the guide and where it is to judge where the headrest needs to be. It should be crossing roughly in the middle of his shoulder over the collarbone, not so high that the belt touches his neck, but not so low that it's crossing over the tip/corner of his shoulder, if that makes sense? And if it's on or under the arm then it's definitely too low.
Some companies also have guidelines e.g. Cybex say place two fingers on your child's shoulder and bring the headrest down to touch the top finger. Britax say to line the bottom of the headrest up with the child's chin. Whichever seat you get, the manual should have info.
In terms of safety ratings of all the seats, they are much of a muchness. The Cybex Solution X is a much older model than the others, which might be why it's cheaper if it is.
When you say they seem to go up far enough is that based on height limit or ability to raise the headrest? I have a feeling that we had Cybex Solution X (not the i-fix though) for DS1 and I regretted it as a purchase because the headrest definitely didn't extend as far up as the Britax Adventure we'd had previously (which was the older model to the current one) and so I ended up having to move him to a backless booster at maybe 8? I am not sure when exactly. Unfortunately I can't be 100% sure because this was more than 10 years ago, so I no longer know the exact model. He is tall - about 6 foot now - but yes, annoying.
Of the two Britax seats, the Adventure is the basic model whereas the Kidfix is the fancy model. It's more comfortable and has some better safety features, although TBH, the difference between different high back boosters with safety is only ever minimal anyway. You might find your DS is annoyed by the strap in the middle which holds the lap belt, and it's probably unnecessary unless he tends to slouch a lot in the seat. But TBH as children get older, especially boys, they don't like it because it can be tight. You can take this piece out, though, so if that's the better seat all around, then it might be worth it anyway. With the Adventure, make sure it is definitely the Adventure Plus 2 - the Adventure Plus had some issues where the headrest support would snap easily and then it just falls straight down all the time, which is why they replaced it with the 2. I am not sure I would trust Amazon to have the correct stock if that's where you are looking (Z-Line is the Amazon-exclusive range from Britax).
The Maxi Cosi is good for the lap belt guides being very open, which means the seatbelt tends to go into it easily and not end up in the wrong place, which is helpful if you have DC who prefer to buckle themselves and don't like to be offered assistance - I know enough about ASD to suspect this might be relevant 
However, it's possible the max headrest height is also slightly lower on the Maxi Cosi seats. On their website, they have the measurement of the internal dimensions and the distance from seat (bum height) to the top of the headrest is 71cm at max extension, so you could compare that to the Joie Bold, but it's hard to tell how it compares with Britax, who only show total dimensions. Maxi Cosi also have a total height shown but it says 72cm so I think it's measured when the seat is slightly reclined - the base is definitely not 1cm thick! Britax is around 83/85cm total height at max extension.
Britax Adventure also does not have isofix, which can mean that as DC gets into the seat it moves around and covers the belt buckle causing frustration. OTOH a seat with isofix can be too rigidly placed in the car which can make the seatbelt hard to reach.