It seems your theory about your experience of the 14-54 performance being largely down to the sensor is correct.
The AA on the 8Mp bodies really was heavy, it could make a sharp prime soft - the E1's 5Mp unit seemed to have a weaker one ..
I'd still prefer to use the 12-40 Pro for landscape, but I'm thinking that an expedition is in order.
if its like the two I`ve had over the years, you can simply leave it at F2.8 and forget about it unless deeper DOF is needed . IMO it`s the best standard zoom for the system and the extra stop over the 12-45 / 12-100 is more than welcome (need all the light you can get with these small sensors)
Thanks for reminding me about my older lens. BTW, focus on the EM1.2 is about the same as the 20/1.7 (without prefocus), which is fine for me.
the 20 is about as slow as M43 lenses get so not suprised ... I liked the E1/14-54 setup very much, even used it in rough weather for work as it was full weathersealed whereas my 1DS Mk1 work camera of the time (2003) had a non sealed 28-70 F2.8L .
E1 + 14-54 full size image - as you can tell I had the E1 a long time , it was the only 4/3 body I liked, build quality went downhill after that as did colour .. thankfully that colour came back briefly with the EP1 and back again in the EM1-II though the M43 Olys all have had nice colour ,IMO the 4/3 cams lost it from the Horrible E300 onwards

Oly E1 + 14-54 - Classic Oly Colour