Toyota Prius - Power Split Device
A little simulation to help explain the way the Prius' (and I believe all Toyota and Ford hybrids) transmission works. The EV mode top speed is there to protect MG1, which has to cancel out the rotation of MG2 to keep the engine from spinning.
The max speed of the Escape in EV mode is about 40MPH. The '09-12 years have a slightly bigger engine, but the software was redone so the engine will turn off more often. It seems like I have to beg my '06 to go into EV mode sometimes, so a '09-12 would probably make better use of the PHEV system. I looked into doing a PHEV conversion to my Escape, specifically an Enginer 4kwh kit. It would take at least 5-7 years for it to break even, and that's if it would double the overall average mileage (very unlikely).
I would do the EV conversion if you think you can make something that your wife would be happy to drive. But keep in mind, a stock Escape or Camry Hybrid will at least match the mileage your Matrix gets. Then you'll be able to decide if the PHEV conversion makes sense for you. Its really up to you.