While the bomb rack does add a little drag, it's only a few mph (less than 5?) and overall not a major deterrent. The bomb rack does add WEIGHT however, so you'll have about ... er... what was it? 160lbs? For the rack gear. Again, on an 8000lb plane, not too much.
The 190A5 has lesser quality MG/FF on the outboard position. However, these are still 20mm cannon. While their explosive load is a bit less than the other cannons, it is still powerful enough to rip a plane up if it hits.
The ballistics are different, but they do make a difference. You can and will get hits with them.
Don't misunderstand, I like the 2-gun loadout as well, but I will often dogfight with all 4 guns just because it means my snapshots are that much better. I will also dogfight with 30mm in the 190a8. Doesn't mean it will turn the best, but you don't want to get into turn fights anyway.
Use vertical moves. Jink one way, then reverse back, rate of roll, climb, dive, loop, you can be quite agile, as long as you don't flat-turn. The only exception is when you're pulling for a shot and the target turns hard. That's where you'll get in trouble more often than not.
Use your WEP, you have 10 minutes of it. However don't forget that sometimes you'll do certain moves better if you back off the throttle. You may find the gas is a bit light. I like longer sorties, and if I take only internal on the A-5 I usually end up cruising more to conserve gas.
For combat, you don't need alt, but speed is good. I don't mean mach 2.5, but a decent rate of speed (250+) is really required for effective manuvering.
I was once bounced (at 12k) by a higher P-51 (~17k), and saw he was going to simply bounce me til I was worn down. I was climbing up when I saw his dot getting nearer, so I leveled out. When I realized the situation, I basically reversed what he was doing. I dove down then up (low yo-yo) and we would merge vertically several times. I actually wore him down to near the deck and after a very prolonged fight and many traded hits, I scored the kill. The 190a-5 is more than capable of killing most planes in the game. Just not escaping from them. Sometimes you eat the bear, sometimes the bear eats you.