I've written a couple of times on here from being a regular user of the 2x27 on Edinburgh to Glasgow in the 1970s. It was commonly called the "Push-Pull" rather than "Top and Tail".
On odd occasions a 25 or a 37 would substitute for one of the locos, this worked fine, and my fastest speed achieved was with a 37 on the rear. The highest speeds were typically approaching Haymarket or Cowlairs - things were always taken carefully down the bank into Queen Street. One type I never even saw used was a regular 27/0; given that a regular 25 or 37 worked fine it was not apparent what a 27/1 added to things.
The ETH changeover was in significant part to overcome the boilers needing to be run under the fireman's supervision (I think they were still officially termed that, and certainly everybody called them that rather than second man), so they had to be switched on and off every hour to use the one on the leading loco. Bear in mind at this time all internal Scottish stock was also steam heated. The Deutz auxiliary ETH generators put into the 27/2 conversions were a right nightmare and would overheat, and in a few cases catch fire. The 27/2 was always on the Glasgow end of the train so this was a real problem when unmanned heading towards Edinburgh. Additional automatic CO2 extinguishers were then provided, and believe it or not those were also unreliable and would spontaneously activate. Even the coaches initially had burning smells, blown fuses and short circuits at the start of ETH, they were initially ex-WCML but had only been steam heated for a couple of years. This was addressed by more thorough cleaning of the heater elements.
At both ends the service came in just behind the stopping dmu, so there wasn't much point normally in running ahead of time. The best performances were with a slightly late start. I have to say each of the issues I experienced were single events over a lot of use, and got maybe unnecessary publicity. Overall the intense service was pretty reliable, aided of course by there being two locos - running out of fuel on the last service of the day was reported, but that would be one loco, the other would get you there.