If I'm honest, it's not that bad. I mean, it's pretty rough in the back (and middle) even though I've ridden much worse. However, last time I went there in 2011 I got to ride the second car and it was perfectly fine, I remember no headbanging whatsoever. (the same cannot be said about the other ride I took in the middle car)
Still, I'm very glad to see they're FINALLY (as you said) getting new trains as I feel the ride has quite a lot of potential.