Gosh, been through this setup so many times in this game! I have turned into the guy, with nose down for speed, then he just split "S" on me as quick as a cat on a June bug, as I pass under him. I have tried going vertical, with a hammer head turn at top, hoping to get a "ho" on the way down, he turns off and I over shoot trying follow him.
"nose down" doesn't mean dive as fast as you can. As you turn back into him drop the nose a few degrees. This will build speed and give you a bit of vertical separation from the bad guy. You don't want to dive so much that he has the option for a split S. Your not going to be building a ton of speed quick but you WILL be adding to it.
2K is pretty close as well, but turning into him right away would work better than a chase were it really becomes too late to turn into him. Also, when you turn into him for the merge yo get to see what "he" wants to do, fight, bnz, be aggressive, be a wuss

If he is a wuss and stays on his perch and gives up E wearily the merge may give you bigger separation and and an escape opportunity if that is your choice (being low ammo).
Just running eats up your options, merging opens up doors.