E34 540i vs E34/E39 M5 steering box

I find steering of my 540i awful. I really hate how it turns in (central play), so I'm going to fight that. First thing I want to try is to regenerate steering box, than put smaller steering wheel. But still - steering box's gear ratio is really bad in these cars. My idea is to change steering box, than regenerate. As far as I know it's interchangable with E34 M5 steering box and this should be an improvement. How's the situation with E39 M5 steering boxes? Are these plug'n'play as well? What's the ratio difference: E34 540i vs E39 M5?

E34 540i vs E34/E39 M5 steering box