It is normal to feel like you owe something to the company that first gave you your big break into the industry. Having said that you are in the ball park at the moment for your career to really take off. Look at all the opportunitys out there for pilots. As long as you have completed at least 1 year of employment with them there is nothing wrong with leaving them. I know it is hard to believe but they will find another keen person to replace you. Just don't bail on them without giving proper notice.
It all comes done to a balancing act leave too early and they will think you have used them to gain hours. Leave it too late and a potential employer may see you as not being career focused.