OK then, a bit more. I seem to recall that the word on 53 Sqn circa 69/70 was that larger props had been intended - my head-on shot up above indicates there was certainly room for them - but Mr Barnes' book indicates that this was simply left as a possibility during the design stage, and that the option was effectively dropped when the 'fastback' mods recovered the performance shortfall.
That is not to say that all became sweetness and light. Belfast Navs became minor experts on parts of the ODM Vol 1 that their colleagues on the VC10, say, could largely ignore. (In my day, Navs from the Belfast OCU ran the RAF's Scheduled Performance course for pilots preparing for civil licences.) We spent much time between trips working up Regulated Take-Off Graphs for airfields and runways around the globe, to take account of Net Flight Path restrictions and Second Segment problems, particularly where ground temperatures were liable to be at ISA+15 or above. Evidence of how overall performance remained on the edge can be seen in the fact that Bahrain - Akrotiri, essentially an airways route, was accepted at the time as a valid Nav Route Check route. The two major issues I recall were, first, making net Safety Height on climb-out by the time one got to the mountains in Southern Iran, across the Gulf; and, later, dealing with a simulated three-engine driftdown and diversion once over Eastern Turkey. An entry in my Cat Card for 17 Jun 70 tells me I must have cracked it that day! (I only flew the fastback aircraft, doing the first or second OCU course once training was resumed in 1969, and I shudder to think how things had been beforehand.)