Hello, We have be related. Nothing definite, but you mention a James Porter born 8-2-1789, son of John P. Porter.
We have Philip b. abt 1750 who could be John P.(Philip) Porter as many families named their sons John.
Philip had a John G. Porter 1st son born before 1798; (which could have been named after him "John" 2nd son William E. b. before 1799
3rd son James b. before 1800 (which could be your James. Then 4th son was Henry B. Porter, b. 1800 in VA --A Doctor and my ggg grpa who married Mary Burch.
Henry B. and Mary Porter had:
1. Guy Henry Porter I b. 7-12-1835 in New Haven, KY Marion Co.
they had: James Henry Porter b. 7-30-1861
John Jos. b. 10-9-1863; Francis Emmanuel b. 1-15-1866; Guy Thomas b. 2-24-1866; Annie b. 1-14-1870; Elizabeth Ellen Porter 5-16-1872 who m Joseph Charles Hagan; Emily Gertrude b. 7-12-1874; and Theresa b 12-12-1877
5th 6th 7th children of Philip continuing were
Charles Porter b. after 1801
Alfred Porter b. after 1802
Katherine Porter b. after 1803.
The only problem that I see is that you said
John P. Porter married Elizabeth McDowell.
We have that Philip Porter married Nancy Ann and not positive but Researchers think her last name was Sexton.
We do not know anything about David, my cousins hired a Researcher who came up with David but we are still not sure.
Perhaps you can put this information together and we can connect. I do not have families names or information on John G.; William E, James, Charles, Alfred and Katherine.
Hope we can work together.
Dottie Porter Himes