Re: I have made a terrible mistake.
Yep, the Mini-14 can be tuned, but I don't know anyone who does it, or at least specializes in it. The difference is that the AR was adopted by the U.S. armed forces, making it THE gun for service rifle matches, and for thousands or millions of people who want to make it shoot better. That, in turn, resulted in a proliferation of after-market parts and a whole lot of folks who have gone into business solely to improve the AR. (Some do, some don't, but that is another story.)
There is now a Mini-14 Target Rifle; I have not seen one and don't know how well they shoot, but they will be hard put to compete with the AR rifles.