ICT SLB, I think that you are mistaken re the origins of the Dash 8 and BAe146.
The 146 evolved from UK studies into RTOL and STOL. Both concepts had high wings. RTOL was the larger project (200 pax) and used 2xRB 211; STOL was the smaller project (but larger than the 146) and used 4xRB410.
From the documentation that I have, the driver for the high wing appears to be performance with good handling – including steep approach.
A low wing appears to offer lower cost of operation, but the high wing would be better on short sectors in conjunction with a shorter field length – also a better climb gradient in the 4 engine case.
There were noise advantages in placing the engine ‘well’ below the wing, i.e. not like 737.
Freighter studies were based on the larger RTOL project (2xRB211), but the airliner lineage dropped the ‘RTOL’ concept and became the UK Airbus (via BAC 3-11?), the final proposals resembling the A300.
The 146 was a smaller version of the STOL project (108 pax reduced to 80). Subsequently a true military version of the 146 was considered (not the STA), but never built.