Would the course of American history have been significantly changed if the Declaration of Independence had included Jefferson's statement condemning slavery?

Yes; most definitely. The northern and southern colonies would have been divided and, thus, there would have been no American Revolution to begin with. The South would have most likely remained loyal to Britain and the North could not fight on their own.

MoreIt's not possible to say whether the Revolution would have succeeded or failed if the Declaration of Independence had included a clause that emancipated slaves. Though slavery was present in all the colonies, it was most prevalent in the southern colonies, those most dependent on inexpensive labor in a farm-based economy. By the time of the Revolution, many in the northern colonies had already divested themselves of slaves, though less out of a sense of moral duty than out of a need for simple thrift. Slaves, while providing "free" labor, did tend to eat every day, and had to be clothed and housed, however poorly. In the north, a trend away from agriculture was beginning to obviate the need for the cheap labor of slaves.

In the event, the Revolution was fought primarily by northerners, on northern territory, and had little impact on the southern colonies. It is easy to speculate that the outcome of the war itself would have been the same.

However, the ratification of the Constitution would have been a much tougher go if it had banned slavery.

Slavery was a minute issue in 1776. It could have been handled properly in the nation under the Articles of Confederation, or in the US Constitution. Slave states then were a handful. If they refused to join the US, the US would have prospered without slavery. It's too bad the Framers failed to see this.