well, what i focused on was the d600 then fall back to 1.1k then close to d600 again situation.
however, taking a look again at the way you further described it, it's easy enough to do what the spit did. i do it all the time in the DA.
as you merged, the spit made his lead turn (probably fired a burst early out) early, hence you seeing him 600 back as you passed. your initial speed allowed you to go into that shallow climb and increase to 1.1k.
the spit is no slouch in either acceleration or climb. as you went into the steeper climb to do your wingover, the spit was able to cut into your arc and gain to that d600 again.
in all probability, the spit would have been approaching his own stalling point (which is why he probably sprayed at d600) and if you had held your vertical a little longer, you might have been able to drop back down on his flopping spit for the kill.
this is what happens to me in the DA when the guy blows thru the merge, instead of a hard g lead turn, i ease off it a bit and go into a high yo-yo. when i make my 180, i'm diving back down to gain the speed back that i lost, and as teh guy goes into the zoom, i stay level a bit closing then cut up into his turn radius.
i'll often be near stalling when the guy noses back down. this is where my experience kicks in. i'll very often avoid his shot as he drops back down (as he'll be slow and wobbly himself, making it even harder to get guns on me) and when he passes me, i roll back down onto his 6 and if he tries anything other than continuing a diveout, i'll more than likely cut into his turn again and pop him.
if you want we can go to the DA and i'll show you exactly how it's done and what you can do on your part.