So, I guess my question is this, what factors affect research efficiency and what can be done to improve and maintain a high efficiency rating in the above situation.
It seems no one has asked you what your inflation rate is. If you're going through rising inflation then your research efficiency will dimish as your inflation rate rises. To mitigate this you have to keep pumping up your overall research investment each time your inflation rate goes up a tenth of a point.
I've found that building more research centers actually isn't helpful as that means more money is spent on operations - a number you can see on the top left of the main research details screen. Better to have fewer research centers and jack up the research spending so more gets spent on research than on wages and facility upkeep.
Research spending spent on techs that aren't used gets spread between efficiency and tech level. I can actually raise efficiency by decreasing the tech level percentage and applying it to researching techs. Makes the techs go quicker and the overspill helps bump up efficiency.
I still think it's rising inflation that's reducing your efficiency over time and unless you can flatten out inflation you'll keep seeing diminishing research efficiency unless you keep pumping in more cash.