leverage = margin
If you've 30k in your account you can buy 120k (not that you should) worth of stock and you'll just have to deal with a one or two cent bid/ask spread. If you're wrong and need to get out you're in a much better position that way. Now that assumes you really day trade, otherwise your margin requirements go up. I wouldn't rely on the mid-price fill too much, they typically work if the stock moves into your direction, if it doesn't you may have to pay up to the natural (or close to it).