I'm afraid you wouldn't get the same results as a commerical turkey, by far. They have such constructed genes that focus on growth,growth growth, that one you breed in a color breed (whether it been a broad brested bronze or a heritage) it takes the focus on growth away, and you will not get one nearly as fast growing as the BBW.
The comination will be a comination growth of both the birds, probably a lot faster than normal hertiage birds, but also slightly slower than the BBW. The weight should also be a combination of both the breeds. But with the heritage breed in there, it will cap the weight sooner so you will have a healthy birds with less leg problems, and more protection agianst disease. Heritage breeds usual weight is 30-odd pounds as toms and 12-20 lbs hen depending on the breed, and a BBW toms can ge to 60 lbs if you continue to feed them high protein diets, but they get leg problems, and hens up to 35lbs.
Mixing the breeds will probably get you a nice 40-odd lb bird as toms, and 25 lb hens if you give them the market diet of high protein.
As far as your current hen, she wwill probably stop growing once she gets to an age. Mine stopped at about 35lbs. Just make sure you get her off the market diet and high protein and feed her anything from scratch grain to layer feed, as long as she isnt on that high protein she should be ok.
Hens do develop leg problems becuase of such as weight, so as soon as you can, dont let her get to my hens weight of 35. over the years, she developed leg problems, but all of them will, its in their breed to get too big. So as long as you take good care of her, and let her get exercise to keep those legs strong she should be good for many years. My hen finally died last Spring as the age of 6 years.
Good Luck!