>>The British ARB (CAA now) insisted on a fully boosted rudder and an increase in tab authority before BOAC could put them on the British register, for that very reason.
In fact I cant remember now but I think there was a difference in the -300 and -400 as well<<
Yes, and as I recall, this was at the insistance of DP Davies.
Operated the -300 (JT4A) extensively and the -400 less so, and I expect they were about the same....but you could be right, gas path.