Biggest problem I've seen is players that fly with 20mm nose mounted and 30mm mounted 109s - trust me unless you learn both ways and fly both often - you will seriously screw up your aim. I always say - stick to one or the other - do NOT inter-switch.
20mm has its advantages of being close to balistics as the 13mm so you pack a rather good punch, but the 30mm alone can't be compared to even this setup.
Today I took up a tempest for first time flying a craft with wing mounted and I could NOT hit anything - not used to converg being an issue -
also I noticed since I wasn't in a 109 that I tended to "spray" and hope I got a hit, rather in a 109 I would fly up there butt and get a shot under 200 most times. I guess it just grew on me as I tend to fly a K4 then G14 so often im used to balistic's change. It does screw up my aim even after a few tours my aim I would say on scale 1 to 10 is rather a 7.