is it possible to achieve a perfectly straight horizon stitching panoramics beyond where I would normally use 'shift' to ensure there was no bending.
First of all, you need to understand, that "horizon" in panoramic stitching is a concept, not a line and has not much to do with the conventional understanding of "horizon", which is very seldomly useful.
In cylindrical projection, "horizon" is the horizontal center line of symmetricity, This is straight per definition. It is your task, to define, what part of the image has to be on this line, i.e. what should appear as horizontal and straight.
Vertical lines appear vertical in cylindrical projection - if the stitching is correct. *All other* lines will be warped, and that has to be so. In other words: only one horizontal line will be straight and horizontal.
See this
tutorian on horizontals and verticals and this
tutorial on warping.
Only Panorama Tools based stitchers will do this right in all circumstances: Panorama Tools Assembler, Hugin and PTGui.