If you're going to use TST for funding, then the MAIN reason you do it is to trade with a lower capital outlay than trading in your own retail account, with the expectation that you will eventually scale up. You have to justify why you are paying the 35% haircut on the first 30 grand of profits, and 20% thereafter. Pekelo already posted the formula awhile back to compare the "true" account size vs. retail.
The other reason to trade with funding is to maintain a structured environment where you are basically insuring yourself against "blowing up" since they will auto stop you on the negotiated limit, whereas in your own retail account, you can potentially hold the draw until you get a maintenance liquidation, where they will force you out AFTER you've blown up.
So it really depends on what buffer you are required to maintain vs. the amount of lot size you can trade, before you can determine whether there is value in the funding.
Now, if they're going to make you first create a 15k buffer as the op mentioned, then there's no point in trying to "scale up" since you could trade decent sized lots with 15k in your own AMP account, and keep 100% of the profits while paying lower commissions, and you also won't have to worry about solvency of the backer.