It may be worth a try to fertilize it with a little tomato plant food and some compost.

Some will disagree but not all Brown Turkey's are the same. They generally look alike and taste alike, but some are better than others in the flavor category.

Not saying you should dig it up but maybe give something else a try if you have the room.

Thanks, folks, I'll try the tomato fertilizer and compost this year, and if it doesn't help, out it goes.

...or graft other varieties to the existing plant - you already have a great root system