Seems to me you haven't got your aim down yet. When you're diving in with so much excess speed, you can't line up your shot, and you can't land the shot.
Set your convergence out further, and open fire further. I'd also suggest firing MG131s with your MG151/20s. They're almost as strong as 50cals, can set fuel tanks on fire, wound pilots, or just give that last straw that broke the camels back (broke the target's wing off). You have plenty of ammo, so fire further out. If you're not hitting, lead more. Take what you think is normal lead, and double it. Doesn't matter if you're too far ahead, they often fly right through your bullet stream.
Now about the setting up of the shot. Don't just dive right down onto the target. Chances are it's heading in a direction other than yours, and as you close so fast you won't get a shot. Instead, throttle back in the dive (to slow down), do a bit of a spiral, until you're behind the target more, THEN finish the dive directly on the enemy's 6 oclock. That will give you the best chance to finish him. If he breaks, then you keep going, climb back up and repeat.
low, slow, and outnumbered... First, if the enemy only show up at 2-5k you don't need to waste time climbing to 14k. Just climb to 8k. If you're not sure how high the enemy is, go higher if you like, but if you know the enemy is low don't bother going sub-orbital. You've got alt and speed. You make a few passes on the targets below you. Keep an eye out for new enemy targets coming in high. If you see any that look like they're coming to pick you, just break off your target and head off to your field, calm and level.
ALWAYS know which way is "out" and which way is "in".... More than once I've RTB'ed to an enemy field and been swarmed to death. Know in general which way is "clear" to head off to.
So, if the high cons look interested, just angle away from them. Stay level if you're in no immediate danger. Most often this is the best way to avoid 5 vs 1 situations. Most times you pull away from the busy cons and only 1-2 want to follow you, making it much easier to engage/disengage as you wish.
If you're slow, level out and hit WEP. You have 10 minutes of it. You won't be using it when you're BNZing, so you should have plenty left to spare. It won't take long until you're fast (and fast and low is much better than slow and low).
I've got an hour-long film that is 5MB zipped, but am trying to find a place to host it. It's a 190D sortie (vox/text removed, for the protection of ... well everybody else from hearing what my squaddies and I talk about

), and I think it's got about 6 or 8 pilots in 190Ds (I'm one of them). We go out and get some kills, RTB, maybe hotpad (can't remember) and we end up finally landing after a long sortie.