Nice post Slade. Don't listen to anyone who gives you grief for extending once you have lost the advantage. That's perfectly acceptable. You also did the right thing by not HO'ing on the secondary merges.
I disagree slightly in the advice given above by Saxman, just to the extent where he says don't fight a corsair fast. Yes, it performs well at mid to high speeds...but so does your yak. If you merge with both you and the corsair above 275mph, try to beat him in angles on the first turn, possibly even the second. But stay fast. The corsair will see you going angles and will most likely think you're playing into his game. He will drop flaps, maybe chop throttle, and do everything to get around on you first. If he doesn't go for flaps, well I'd say given equal pilot skill a mid speed yak and a mid speed corsair should make at least an equal fight, if not slightly in the yak's favor for reasons we will see later.
But let's assume the corsair does drop a notch or so of flaps and maybe has cut throttle. If you beat him in angles on the first or second turn, well congratulations that's it right there. If (more likely) you don't, that's fine, because now you should (granted you didnt chop throttle yourself) have the option to go vertical. It takes practice to know when this moment is, but you'll get it. Ideally wait until his nose is pointed away from your direction of flight, otherwise he may still have enough E to nose up for a shot. You should be able to get the perch easily, due to the corsair's earlier throttle and flaps manipulation, along with its poor (compared to the yak) climb rate.