Howard has the right idea, a piggy backed battery or a software hack to access the other 3-4kwh of power in the battery that is always held as a buffer by the software would be a good idea. I like the idea of a 20-25% power reserve. 50% is overkill.
I made an inquiry once: here is the response from: [email protected]
I have no idea what is the Honda battery. If is lithium batteries with max 4.2V per cell - could be replaced with more powerful directly - we can offer Nissan Leaf batteries for that. I could not find any info on internet.
For example - Prius EV 2012 has 20Ah batteries - 56 pieces, that could be directly replaced with 28 Nissan Leaf modules - and that will increase the range from 13 miles to about 40 miles. That is relatively easy job, because both using same battery chemistry. Cost is abut $5500
I don't know about the Honda.
You can check with Phuginsupply.com - they could have something.