Hi Wes, Welcome to AVS!
I think you saw a difference at Century Home Entertainment because of three basic variables. First of all it was a 1080p projector, then there are the darker walls and ceiling, and finally the Stewart Silverscreen. I also suspect the room was fairly dark too.
In your case you have light colored walls that will cause a loss of contrast due to rebound ambient light. A white surface is most effected by this. If you want to get a similar image then painting the walls a darker color would make a big difference. It is not necessary to paint the entire ceiling a dark color. It is the area of the ceiling directly above the screen that will cause the most trouble. Here is an example of what you could do:
I would recommend the Behr 4850 + Minwax Satin Polycrylic over a satin finish paint. Not just for the added gloss from the polycrylic but also the benefit of the smoother finish. If the wall already has roller texture, you will need to do some wet sanding to get it smooth again.
The Valspar Flat Enamel is also a very good paint that levels out nicely. I would recommend you take it back to Lowes and get them to tint it True Value "Winter Mountain" or Glidden (ICI Paints) "Universal Gray" 00NN 62/000.
The Epson 720 in Living Room Mode used with an N8 gray screen should produce a nice rich high contrast image in the presence low ambient light. The gloss level of the Valspar paints is a bit lower than the Behr paints so you may be ok with the satin. The concern would be that you will get some hot spotting. I can't say for sure since I did not try the Valspar satin enamel. If you could return the satin and get the flat enamel and add 8-10 oz of Minwax Satin Polycrylic, then I could say with some confidence that you will get excellent results.
I would start with ensuring the wall is smooth, then paint the screen with the N8 tinted paint. If you are still dissatisfied then consider painting the ceiling directly above the screen and the walls a darker color.
Here is a simple demonstration of the difference between a white screen and a gray screen:
In the dark:
With some ambient light:
I have no doubt that the Stewart screen is excellent but it will also be expensive. Sanding your wall and applying the paint properly will also provide excellent results for a fraction of the cost. All the other factors such as room colors and ambient light levels are the same for both a painted screen or a commercial screen.
Review this demo video to ensure you are apply the paint properly:
I have tried to summarize all this information in the Beginner's Guide To Simple DIY Painted Screens
sticky. Take a look through that to get a more comprehensive overview of the basics of DIY screen painting.