Center wouldn't be of any specific added benefit other than dividing the total weight by 5 instead of 4. That's why most things with feet aren't designed that way. Also the only horizontal steel across the bottom of these cabinets is thin sheet metal. The whole point is to have the least amount of surface area between the cabinet and the ground. I gave this a lot of thought as to how much weight will be in these cabinets, the weight distribution, and the points of the most strength. The only thing "better" would be steel all the way around the perimeter of the bottom, but it would be expensive and mostly pointless. As far as the legs themselves, you might not be familiar with 3/8 inch bolts. It would take some extreme weight to bent 4 bolts of this thickness and length pushing straight down on them.
Only things I might be worried about if anything are:
1. The threads in the plates not being hardened steel and wearing out. (As long as they aren't being screwed in and out very often, it's probably not a problem.)
2. The washers added to the bolt heads bending when the cabinet is tipped. (It's probably unlikely with the cabinet empty.)
3. Even with the washers added to the bolt heads, there still being too much concentrated weight on the carpet in that one area. (Once I have lived here for three years, I'm no longer responsible for the carpet.)
4. Kicking the front legs on accident, wiping the Fluon off. (The Fluon added to the surface of the plates should remain pretty much untouched, even if it's kicked.)