Well, a 254 is not really a 50cc class saw to begin with. Like the 2055 Jonny it's in what might be called a mid-50's class. Kind of a half size up I suppose. (Not suggesting we need another class either.) A good running example of either will outcut any of the 50cc class leaders. Both respond well to opening up the muffler a bit.
The 51/55 are hard to pin down as to being consumer vs pro. IMO, the sentiments on this site are too reflexively negative anytime we are talking "open port". They were marketed to both pro and homeowner, but they mostly existed at the same time as the 254 which was a true pro saw, we have that perception that one is pro and one is not. But often that line is somewhat blurred.
I totally agree on the 254xp, no problem at all. It simply is a bit large and heavy to fit in the 50cc class, and the power fits well with that. Btw, the last time the official German test lab DLG tested one (or more?), they "found" 3.1 kW at their dyno (4.2 hp) - but I doubt that was typical of them? That is the same as the specs for the 555....
The 51/55 doesn't fit in todays pro vs. homeowner "class" system, because they were concieved from the beginning to be "Rancher" saws, by the standards of the 1980s, with a magnesium case. As we all now, todays "Rancher" saws doesn't live up to that at all (different story).
Some random thoughts (and notes) on the matter, not related to your post;
If we call the 53.2cc 55 a 50cc saw (we do), we should equally call the 2055 one the same, as the cc is exactly the same 53.2cc (except for the cp 55, that was 50.9), and the weight very close. The 2051 (also 50.9cc) should be the rather uncontested 50cc champion of the late 1980s and early 1990s, and it was replaced by the very similar 2055. The 254xp is only .8cc more than 53.2, but it is a larger, heavier and stronger saw - too much so imo, to fit in the class.
A "joker" here (in the historic perspective) is of course the 115iH, since the original specs were quite impressive - however, the issue is that DLG tests showed it weights substantially more than the specs told, actually more than a pound more, bringing it up to 2055 weight. An interesting note is that model still is listed as a current model in Europe (well, at least in Germany and Norway, didn't check any other sites).
The info is a bit confusing regarding what the closed port 53.2cc P5500 and PP325 weights, so it is a bit hard to rate them against other saws - the only somewhat reliable info I have indicate they were quite a bit heavier than the P5000, and even heavier than the 254xp - but I don't really understand why they should be? Power rating was very good in the 1980s (at 3.6hp).
Then there was the 026, that never was the "power champ" of the class, but is lighter than all the above?
First, it never performed as well as the rated power indicated - second, the inboard clutch offset the the advantage of the lighter weight - and then came 1999, and a few years later the US version of the MS260 (sad story)..