As well as the Akerman that John mentioned the other thing you need to consider if you swap hubs "side for side" is that the caster angle is facing the wrong way. You can design around this but it means some difficult wishbone angles, this is the main problem in using the Vauxhall Omega as a donor.
In answer to the perfect upright question - there isn't one. All suspension design is a compromise based on weight, required steering effort, principal use of the vehicle etc. It's a complicated process and the ability to design your own upright is part of the process that is a good thing if you can do it as you can optimise that part of the design. Usually you have to work around the limitations of the upright you're stuck with.
I've been forunate in designing the single seater so that I have an optimised upright for that particular vehicle. It might be that that upright is better for the Roadster too, it won't be optimal though as it's not purpose desugned but it might be an improvement.
Cheers
Chris
