Does paper mode use my real account or a separate one?

Good question, and the distinction matters. Paper mode signs in to your **real** account for market data, so you see exactly the prices and the depth you'd trade against. Balance and fills are simulated inside the app. No orders are sent to the broker. That means your test uses live market conditions rather than a demo environment's approximation. The one thing simulation can't reproduce is queue position on a real order, so treat fills on very thin names as optimistic.

This is why I trust paper results here more than the demo-server kind I've used elsewhere.

Worth keeping in mind that even with real data, the fill model is still a model — simulated fills can't know your queue position, so anything resting at the touch will look better on paper than in practice. The gap tends to widen with size and in thin books. Are you planning to test at the size you'd actually trade, or scaled down?