The Xspeed ZX is a small, really light (1t) midengined sports car, powered by a 2.0l inline 4 turbo.
Xpeed buys its engines from Nero, so the ZXS uses the engine from the Nero 700S, but mounted as a midengine.
The ZXR is the more trackfocused version, that has the engine tuned by Xspeed to make 55hp more by increasing the rev limiter, the turbo pressure, the wastegate, the cam profile and other stuff. Too much for just 55hp? Yes! But that's just it.
It is also lighter, has better and stiffer suspension, more aerodynamics going on, wider and grppier tires, sits lower etc...