The sweet tart that I've had were firm. Not quite the firmness of a pickering, but still fairly firm. And, they can sit on the counter ripe for several days without breakdown.
If that's the level of firmness you're looking for, cogshall may not cut it. You can eliminate jelly seed by cutting mature green, but it still isn't a firm mango.
Sweet tart does seem to enjoy growing :-).
And, yah, if you decide to delete the cogshall, topworking would be recommended. You get a huge head start on growth by topworking a mature tree. If you're cogshall is currently 18 feet tall, with 2 full summers of growth (apr - oct) your topworked tree will grow back to 18 feet :-). A top-worked tree grows with incredible vigor as it tries to get back to its previous size.
Another thing you can do -- if you're an adventurer -- is topwork a seedling to a drastically cut back tree. You can shorten the time from seed to fruit dramatically by doing so. So, if you had, say, a maha chanok seedling that you wanted to fruit, cut the cogshall down to about 5 feet tall and top-work the sprouts. You'll get fruit within 3 years from the seedling vs an 8 year wait if you were to plant it :-)