Watts/sqft isnt ideal, but for people getting into growing, its a useful way to get a general idea of what will work and wont. One of the biggest problems new growers have is growing plants under inadequate light and wondering why they dont develop properly. Of course some of the new generation LEDS produce much more light/watt that older ones do, but for cheaper level LED's, I think it gives people a good target that should allow them to grow respectable plants. A 900 in a 25sqft area would be too little IMO, about 18w/ft, but Im sure some growers could still grow respectable plants using this light. At the same time, others would struggle and end up with fluffy buds....
BTW I just noticed the OP was talking about TWO 1600, that is a lot of light for a tent this size and unnecessary unless you are running a serious SOG under ideal conditions, probably need CO2 also to utilize this much light...
If you are sold on Mars2, Id consider 4 mars 2 400s for this space, that would be really good, or look at the reflector series, while I havent used these, Ive seen enough grows using them to know they are decent lights and I like the ability to switch off some reds during veg phase.