It's not the engines that would prove limiting to cruise speed at those altitudes, it's the aerodynamic design of the wing. Airliners are flying at between 78 to 86% of the speed of sound, you can't improve on that in a way that would be dramatically noticeable without a huge redesign.

https://en.wikipedia.org/wiki/Swept_wing#Subsonic_and_transo...

Many 757s have RR engines, but adding more power would allow take off with higher weight from shorter runways etc, not faster cruise.