I've had the piece of foam blocking about 2/3rds of the right side of my airbox for about a year now. The bike does indeed run a little richer, and I've seen a slight drop in MPGs, maybe 1 or 2 mpg off of 45+ MPG. However, the low end torque is much nicer now, and the bike handles much better at lower speeds.
I do NOT have Steve's jet kit yet, but am trying to save some pennies...
I have also noticed a small drop in high RPM responsiveness (when I'm canyon chasing with some buddies), but it is very easy to simply pull the foam out if it really bothers me. That being said, I've had the foam in place all winter/spring.
I'm also at about 4250 feet in altitude, which plays into my mileage figures a bit. I did some research on altitude versus MPG, and apparently in a lot of cases (Connies included), MPG generally increases with altitude (because the engine doesn't work as hard to draw air in, etc). Of course, we also have about 10-15% lower HP at my altitude as a result...
I've also noticed that my #4 plug indicates that is running slightly richer than the other cylinders. This would make sense, since the right side of the airbox is the side with the restriction.
Short form, yes you can run the 2 minute mod (foam in the airbox) without Steve's jet kit, but it is highly recommended to get the jet kit to get the full benefit of the mod. Steve's jets are tuned to take advantage of the new airflow characteristics, resulting in increased HP/Torque, MPG, and/or both.