Bit more feedback on this…
Raised the front axle a bit further, to ~35mm. On a 1010mm wheelbase this ~=2 degrees angle, equivalent to a mild ~3.5% road gradient.
This felt good. Legs feel very much like they do after an outdoor ride. Hip flexors don’t feel like they were being activated so much (ie. too much). Glutes and hamstrings felt like they were able to do more work and it felt good.
I’ve mentioned here before that I mainly ride in very hilly terrain, with limited flat roads; because a lot of the hills are very steep (pace slow) this means that time-wise I spend a good proportion of my outdoor riding going up hills, ie. with the bike tilted up to varying degrees.
Therefore, it makes some sense for me to replicate this indoors, and it’s not a surprise that setting the trainer bike up to better mimic my common outdoor riding position (front axle raised higher than rear) feels more natural to me and any adaptations I may have developed from forever riding hills.
@mcneese.chad’s theory re aur pressure forces sounds reasonable, but as Chad says it’d need tunnel time to test it; my use-case here doesn’t depend on that theory holding up - tilting up simply better reflects my position outside.
I’m glad this thread started 
because on current evidence (just a couple of hard threshold Workouts so far…) this simple change of raising the front 1-1.5" seems to be suiting me very well and it feels much better and more ‘realistic’ than previously when axles were level. 