Anyone tried a 140mm lefty on a rush – be interested in this for the downs
might mess up the ups ofcourse!

Yes, it’s entirely possible, although you’ll exceed the maximum rated fork length for the frame.

I have a lefty max on my rush with a spacer to take it down to 130mm.
That way it’s 500mm axle to crown (max for frame). Plus I think it would be a touch slack at 140mm for ‘normal’ trail riding.

I think it’s improved the bike loads. Not quite as good up hill (but still fine) and much better down. BB is slightly higher too (13″) which I think is better.
Incidentally, the geometry with the 130mm lefty is the same as the Rushes speced with 120mm fox forks.