Aero question...
 

Should I take the spoiler off?

http://i917.photobucket.com/albums/a...s/GEDC0145.jpg

Looking at pics of other Souls, it's not a standard feature, so it most probably doesn't help much.

Those without a seperate spoiler like yours seem to have a small integrated spoiler though. Is that also on your Soul ?


Hint : delete the mud flaps :)

Not sure about the separate spoiler part..

Would deleting the mud flaps help that much?

The mud flaps are there for a reason, they will reduce your "wake" in rainy weather and wet roads. You would be better off boat tailing them... I removed mine (because they were old and brittle) and saw no improvement, I have been thinking about boat tailing the wheels, but haven't gotten around to it yet.
Removing the spoiler should help, replacing it with a Kamm-type-back would help, check out what Weather Spotter has been doing with his vehicle... you both have similar hatch setups, you may be able to leverage off his experience.

Most mods won't help much - you've already done one of the most effective : the front grill block.

Anything may only help a little bit on its own, but all those little bits added together start making a serious difference.

If you look at my fuel log, it's all down to changes in the way I drive, less AC use, some weight reduction, and now the grill block.
I used to be happy with the rather decent 43mpg I got on the first entry - but the last entry so far is over 52mpg :)