After several years of growing many varieties of organic heirloom tomatoes, I am concluding that heirlooms are simply sparse fruit producers in general

I would strongly disagree with that conclusion and I'd suggest that you first detail and re-evaluate the growing methods you are using since that, and not the variety, most determines production. This given the fact that many growers, especially in CA, are quite successful growing heirlooms.

I'm also curious about your use of the term "organic heirlooms"? The two terms are not synonymous. Do you mean you are gardening organically or only buying organically grown transplants?

You might also want to read through the many previous discussions here on grafting. Like everything else in gardening it has its pros and cons.

Meanwhile check out this how-to graft tomato plants link.

Dave

Here is a link that might be useful: Grafting tomatoes discussions