Your EB mileage pretty much jives with mine - 17.5 is my average over the last 12,000 miles. It seems that if you drive sedately or hyper mile the EB engine it'll adapt to that, then when you need it to crank to and punch it, it has problems.
Best thing is to keep it blown out regularly (like every time you drive it). If this is done, it'll always run fine.

How do I know? I got mesmerized trying to get the best mpg possible and actually got my FX4 up 19.9 mpg just taking it easy. But while all this was going on I noticed that the truck just didn't feel "crisp" any more and then, finally, while merging into traffic one day she got short winded on me when I opened her up. For several days I was perplexed over all this.

Then while out in the Texas hill country, I decided to blow off mpg's and just flog the truck to have some fun. Every grade I had to climb was done in passing gear, full bore until I had to let off. I caned my truck for 45 miles like that and when I parked, I had my old truck back. All those bad habits GONE!

Now, I just drive it without regard to mpg's and the truck has settled in around 17.5 - exactly what she used to do before my hyper mile madness. Enjoy your EB truck, just remember, baby loves the whip.... a lot!