It wouldn't be as much better as you think, and probably much worse. Some thoughts:
An engine isn't most efficient at its maximum output. Here's a BSFC (efficiency) chart for my Honda:
Even though the engine can output 67HP, its efficiency is terrible there. It's actually most efficient producing ~17HP, or approximately one quarter of its max output. A 14HP diesel would probably most efficiently produce far less power.
On top of that, the efficiency of Honda's and Toyota's recent gas engines (particularly in their hybrids) is very close to that of the best diesels. Diesel fuel has a bit more energy per gallon, but it's also usually correspondingly more expensive. So in terms of cost per mile, it's basically a wash.
You already know this, but your car already uses an electric motor to aid in acceleration, and the transmission does its best to keep the gasoline engine in its most efficient range given the power output and capacity limitations of your battery and electric motors, and the acceleration you're asking of it. If you want to keep the engine from revving into less efficient ranges, simply accelerate more slowly. The Prius already does exactly what you're asking of it, and the engine is already close to the best out there, and is appropriately sized for the car - much smaller and fuel consumption might actually go up. Improvements over what you have will come from things like weight reductions, lower rolling resistance, improved aerodynamics, and having a larger battery which can be charged from the grid. And, FYI, there are plug-in conversions for your Prius:
https://parts.olathetoyota.com/blog/...-plug-in-prius
Be aware that electricity is not free. With low gas prices and high electricity prices, it's generally not
that much more economical to drive on electricity. Sure, you're not burning gas, but you'll pay for it on your electric bill.