I actually share your suspicion. I believe that the impact on performance as you approach the extremes of most of the relevant ratings is something closer to logarithmic than linear, but my math skills aren't up to the task of confirming it. I just think you need to get to 80 - 85 before it becomes noticeable.
Would i start him at COF? It depends on the alternative. His defense is about average in RF, but will prevent a few runs above average in LF. If i can use the salary i'd save playing him at a rookie contract to get a significant boost elsewhere then i think it could work.