Hard to say I guess exactly what Sykes and Fairbairn would have said today. They do eliminate the .22 and .25 as legitimate choices right off, but consider the .32 and .380 Autos as apparently legit.
I do know that Fairbairn armed the smaller-statured Chinese members of the SMP with Colt .380 autos rather than with 1911s.
His own preferred weapon was the 1911 (and I sure would like to find the text of his 1925 book on the use of that fiream), but in "Shooting to Live" him and Sykes do write favorably of a cut-down Colt revolver in .44 Special.
They also allude in "Shooting to Live" to not being allowed "by the rules of the game" to use expanding hollowpoint ammo.
What they meant by "very high velocity" depended on their frame of reference I guess. The standard velocities of British revolver rounds were exceedingly weak, even the vaunted Webley .445 lumbering along at around 650 fps, and his .45 ACP 1911 with 230 grain ball probably had an mv of somewhat less than 850 fps.
OTOH the .30 Mauser/7.62 Tokarev round they mentioned as being a paragon of high velocity used a 90 grain bullet travelling at a mv of 1,400 fps (and was later dropped by the Germans who created it when they expanded the neck of that bottleneck case to give us the familiar 9mm cartridge we still use today).
It might be that today's hotter/lighter/faster .45 acp bullet options (ie 185 grains at about 1,000 fps) would have also fit their criterion as a "middle ground" option, I dunno.
One theme that does stand out though is how very early these guys adressed the familiar questions we still debate today.
Judging by his writing in the "Stopping Power" chapter of "Shooting to Live", Fairbairn likely never would have said of his 1911 that "they all fall to hardball" and apparently did not share either Cooper's or Clint Smith's degree of faith in the efficacy of that round (but lets face it, by the time "Shooting to Live" was published he had accumulated far more first-hand practical experience than both of those guys combined).
Interesting that in "Shooting to Live" they specify that one must respond within a "third of a second" to a threat in a typical "affray", a requirement which seems to exceed possibility if one includes normal human reaction times.
Elsewhere (maybe in his 1925 book) Fairbairn had written that a typical altercation involving deadly force would occur by surpise, in the dark, within ten feet, and that the outcome would be decided within the first two seconds.
I have long kept those maxims in mind when conducting my own practice at the range.
Birdwatcher
