I can answer one question for you having gone from a HD350 to an X35: The motion is much better: I never was that bothered by it on the HD350 as others seemed to be, but I have noticed the X35 is smoother (I don't use CMD either so it's not that). I can't watch 3D due to headaches, so my comments are purely based on 2D.
I saw a noticeable improvement in contrast, but this is partly because my HD350 has less contrast than your HD750 to start with plus my lamp had dimmed some, so I had the iris well open and now with the much brighter X35 the iris is fully clamped down for 15fL so I get max contrast. I would argue that apart from the brightness/aperture change you would see much less improvement going from HD750 to X55. I would suggest that you might need to consider the X75 as it has more native contrast (for 2D and 3D) and you will gain on the aperture as per the X35/55, but it isn't any brighter than the X55 (the X35 is slightly brighter than all the other models since it doesn't have the Eshift glass to pass through).
I don't think that the Eshift is such a huge difference, but others swear by it. More that the MPC gives lots of options to increase apparent contrast/shaprness. Personally I'm not keen on MPC set even to default settings as I find it bit too much for my taste (same as for the Darbee if set above 50%, it just looks too processed to me and I could see ringing on certain scenes). Having seen a true 4K Sony VW1000ES the JVC Eshift is a poor relation (but then I'm too poor to buy the Sony anyway
My comments should also be tempered by the fact that I chose the X35 over the X55 because in the UK if you by the 2D version (no emitter or 3D glasses) then the X55 is twice the price of the X35. I already had a Lumagen video processor, Darbee enhancer and an Isco lens (for higher pixel density with 2.35:1) so these 'accessories' IMHO help to boost my X35 to close enough to the X55 in many areas and better in some (the 125 point CMS/ autocalibration).
Just by the by, II consider that our HD350/750 models are 4 generations out of date if you follow this timeline:
HD350/750> HD550/950> X3/X7> X30/X70> X35/X55