Rolls did a couple for BEA pre-Viscount to get some operating experince but they were un-converted after a couple years.
Conroy Aircraft put Darts on a Three and Super Three around 1970. They later pulled the Darts off the Three and put on PT-6s, don't know what happened to the Super Three.
I'd guess the reason the performance numbers don't change much is the power plants are staying in the 1200-1350 HP range of the recip engines. No one seems to want to do the engineering to up the power plant. What you do get is a bit more cruise speed - less frontal area - and a whole lot less maintenance.
Good start Dufo, but the conversion of what number?