Thanks for quick respond.
I appreciate that it is not simply a matter of longer rods and bigger box and build plate and must admit that my gut find the rods indeed a bit thin for such lengths. Some Chinese manufacturers though (up to approx 400 x 300 build plate) have done this, look a bit like a stretched UM and seem to work and are being sold (legal issues?) even though some reviews are a bit 'scary'. I find it a bit strange that Ultimaker, considering its commercial succes, has not entered this avenue (albeit as a kit with stronger rods, etc) as it is a logical fact that this too will find a huge market closing another gap towards hard-tooling.
I am building relatively large flat 'trays' and despite your arguments about a large printer being 'freakishly slow' (you are just building bigger products, print speed may be slightly less considering added inertia) I think that I would gain massive time (and simpler and more reliable design) printing them in one-go instead of all the warming-up, cooling down, gluing, tolerance issues and whatnot.
Note: I bought an UM2 based on specs re. build surface of 223 x 223 which turned out to be a mere 195 x 195
So from your much appreciated reply I understand that no efforts are being undertaken but if I am wrong I would be more than happy to hear from Ultimaker (community) or others on what the status is. Meanwhile I will have to conclude to go on outlook for an alternative.