Congratulations on your nicely growing heirloom tomato plants. Just transplant them as soon as you can and they should be fine. Yes, tomatoes are technically fruits but there is also a tomato forum where you might get more tomato specific feedback.

I should clarify, by growth I am primarily focused on yield. and by yield I mean many reasonably sized fruits rather than one or two enormous ones. This will not affect their yield in any way even though it will be too cold by October for them to continue growing?

I would suspect leaving them in a small pot will definitely affect yield. I would think you would want them in a permanent pot long before they fruit.

The transplant results depend largely on how many of the roots your potted plants have touching the pot (rootbound).

How large are the pots?. Are you transplanting into the ground or into another pot?

P.S. Are you referring to the leafy herb lemon balm or the tomato Lemon Boy? Try the herb forum for the lemon balm.

The bottom line is that you want to get them in their permanent home, whether in the ground or in a large container, as soon as possible, and hope for the best. You still have at least 2 months of good growing season left.