the 57 contains similar features as the sl1 but does not necessarily contain an sl1 inside it. the same similarities are shared with the sl3, only major difference would not be the fact that the 57 has a 1.0 usb and that sl3 has a 2.0 usb but that the sl3 has a 3rd deck to be used thru the aux line. i mentioned the usb 1.0 and the 2.0 because that was one of my main concerns since i switched from an sl3 to the 57 but "speed" wise it make almost no difference unless you are very picky about tech stuff. i was exited about being able to use 3 decks at once but when you think about it, if you are mobile you would have to carry an extra piece of equipment and you might not even use it as much. it is just not me ofcourse. i spin just about the same genres and as u, i am not a battle dj but i love the fact that you can do so much with a 2 channel mixer. i have gotten experience using a "3rd deck" by simply pluggin a cdj to the aux or one of the 4 channels i am not using to control ssl.
one thing that you would be downgrading would be the midi controls from the 57. if you are used to using the buttons to scroll thru tracks or even just drop to the cue points, you will definitely miss it. from experience i have found myself looking for those buttons on 3rd party mixers, it throws you off because you might be so used to using them that it might slow you down on how fast or accurate as well as comfortable you mix.
i have only tried the demo but the video plug in is great when looking to expand to video mixing. this could be done withe sl3 as well but again you would lack some features that are built in to the 57.
one thing that i would recommend you get if you do decide to change your gear would be the dicers from novation. even though i still use my 57, the dicers allow some crazy things to be done. you have all ten cue points at the tips of your fingers, loops, loop rolls, the built in effects and even more depending on how you map them in midi. it helps with the transition of not using the 57 and using something else instead

if you get the 800 you need to get the innofader. the stock fader is crap IMO.

no need of getting the innofader unless u scratch often

unless he scratches often... the stock fader will be more than enough if very little scratchin is done