I'm wondering if Interactive brokers demo account is so realistic that the demo orders would have the same slippage and so on, like live trading accounts at IB would have. Regards
Demo account is unrealistic in many ways. It is only good for an into to TWS and basic API testing. It is definitely not a market simulator. IB paper trading account is fairly realistic.
It really depends on what you are trading. If you look at you fills and see if at any point you were the ask gets to the level of your buy limit order or the bid gets to the level of your sell limit order then you can count on getting the same fills with live trading. If you are testing any scalping strategies that involve you getting to the front of the queue your results will be suspect.