Airplanes are designed to meet many requirements that are influenced by speed.
One of them is flutter, but there are numerous others.
One important one is design considerations for gust loads.
Likely all pilots know that in turbulence, slowing down reduces how rough the ride is. Inversely, the faster you go, the worse the ride is.
Airplanes are designed to tolerate a specific gust load at max structural cruise speed. If you make modifications that change the max. speed capability, but nothing is done to the structure, you would be at risk of structural damage or failure because of gust loading while flying at the higher speed.
Any opinions expressed in this message are my own and not necessarily those of my employer.
RV-6A (aka "Junkyard Special ")