Sorry about the late response Nick (been out of town for a while), i guess the answer is that, all base systems will deflect somewhat, and the real question is what are you willing to accept (remembering that the SO3 is more of a bench top hobbyist machine than a commercial machine). Should you prefer the 80/20 extrusion system and can accept the additional weight penalty over the supplied bed, I would recommend something a little beefier such as 3.00" X 3.00" or 3.00” X 6.00” T-Slotted Profile, or even contacting https://www.8020.net and getting them to design and pre-cut the product (if you don't mind the additional cost).
All my projects are in aluminum and as such, i need a reasonably rigid bed. For larger jobs, I manage the deflection by temporarily bracing the underside of the extrusions to distribute the load, and adjust the cut settings to achieve repeatability and tolerance requirements.
Hope that this helps.