If you want to get more speed I think going with a friction roller drive is your best option.
I am considering doing this with my Priority Bicycle for added speed especially up hill.
These electric systems are easy to install and use on any bike and can easily be use on many bikes and don't require any changes of bike components.
It just adds to the bike without sacrificing anything and gaining incredible speed and up hill with ease and no sweat.
These friction roller drive kits are basically an electric roller that rubs against the wheel created friction which then makes the wheel move and the faster you peddle the faster you go.
It gives you super human bionic speed.
Here is one friction roller drive kit from a company called
add-e: Simply add electricity to YOUR bicycle
Don´t buy a new e-bike. Instead, transform your existing bike into a powerful e-bike within seconds
Here is the link to add-e with video pictures and description
https://www.indiegogo.com/projects/add-e-simply-add-electricity-to-your-bicycle/x/8433777#/
I hope this helps you out.
If you use this option I would like to have an update to see how you are doing liking it and your experince.
Well enjoy and have a great day.
Hope to hear from you