Just to back up what Jazz is saying.
The Shapeoko and Workbee forums are full of posts about upgrading to linear rails and ball screws to improve performance and reliability. Carbide 3D now offer an HDZ Z axis (linear rails and ballscrews upgrade) for around £500. Interestingly that was developed by a user, who also started selling plates to allow an all LR/BS upgrade which would have allowed a kit to be assembled for around £2,500...but he was recruited by Carbide 3D as their head of product development... HDZ frame no longer available but some await their next product announcement with anticipation. Meanwhile the i2R and recently announced Hammer 47.82 look like proper machines and cost about £3.5k....but their specs indicate significant corners had to be cut to get to this price point. So there are plenty of people trying to address this market but it is all about compromises.

For £2-3k you can buy the required components to build your own machine to your own requirements... that would cost £5-10k if sold complete due to labour, tax and customer support. But the time and effort should not be underestimated. I am reluctantly accepting that perhaps I will have to design and build my own machine as the market will only be able to offer affordable but compromised or capable but beyond my budget! JazzCNC keeps dropping hints, we can just hope to be patient enough for him to make his play.