I'm beginning to think that the original sample rate is somewhat irrelevant. Many of the 16/44.1 MQA releases, encoded from CD masters, are up there with the best HiRes that I've heard on SE loudspeakers.

There are so many variables, but consider an early AAD (analogue recorded, analogue mixed, digitally mastered) release: the MQA encoding will essentially be undoing the (considerable) imperfections of the ADC that was used to digitize the mix tapes. THAT is the main reason why MQA encodes sound better than their CD masters. (Plus there is the correction at the DAC end as well.)

Some red book CD sounds great, but much of it doesn't, whilst a lot of numbers game "HiRes" digital sounds great, but certainly not all of it. Ultimately, a great recording at 44k will sound great and a lousy recording at 192k will sound lousy. The MQA encoding process seems to be very effective at extracting the very best from the master, and from what I've heard, it's possible that the benefits are more significant for 44k masters.