"Production was about to proceed when, in May 1957, it was decided to change to turboprop power in light of the success being enjoyed by the Viscount and Fokker Friendship". British Commercial Aircraft by Paul Ellis, Jane's, 1980.
"Development of the Rolls Royce Dart turboprop and competition from the F27 Fokker Friendship forced a redesign of the engine installation to two Dart turboprops." Bournemouth Aviation Museum web-site.
As the F-27 was virtually identical to the Dart Herald the effect of the F-27 on sales of the piston Herald was probably far more significant than the Viscount.