I have a Titan tank and the only problem with it, is that it hangs down an extra 2-3 inches more than the stock tank. It is only a problem if you need the ground clearence.
I have one and like it alot. Nothing in the box. Its a 228 litre tank but so far I always only put in 175 at fill ups. Love to get big range out of it. I fill when I want. As for it being a little lower, I don't think it should be a problem as the axles are lower.
I have a slip tank gravity feed into my factory tank and total it 410L, love the range but it is big and takes up some of the box.
If you go slip tank and need a pump, I have a 15GPM pump brand new never used just installed and removed after driving around a bit.