If I were you, I would get the Grom and find a more fun alternate route to work. Twenty miles on the highway wouldn't be much fun. I do short runs on I-35 and other highways around Austin from time to time but I usually just ride through town.
The top speed can depend on how fast traffic is moving around you also. I've been on the highway before with a LOT of traffic moving 65-75mph. The tunnel of air that starts to suck you along is pretty intense and I've been able to sit straight up and down and see 70+ mph in some cases. Before you big guys say that's just due to my size, I had a certain legend with me that weighs 250lbs (and then some) doing the same thing with me. ;)
You'll see 95+ mpg regularly depending on how you ride it. Smaller people who ride really easy see around 130mpg at times. It's worth getting up earlier and taking the fun way. :)