I dont fly the G14 much at all, i fly the K4 more often than not out of the 109s.
FWIW, the biggest strength any late model 109 has over a P51 is its climb rate. It has a peak 1300 FPM climb rate advantage at about 5K over the P51-D and more over the P51-B. Your best best would be to employ this. Keep the fight in the vertical and use E tactics and you will soon have the pony gasping if he's not too clever.
As for turn performance. The mustang has it in spades all over the 109 at high speeds. Sustained performance is fairly even given the pony has flaps to play with and is probably slightly more stable at slower speeds espc the 51-B. But next to nothing in it really. Pilot skill would decide that.
The mustang is faster than you across the alt range, so dont try run from one.
A few generic 109 pointers wouldnt go amiss either. Watch the speed, 109s have awful elevator authority at high speeds. Avoid overspeeding at all costs. The mustang will romp you in that regard. Get ready to cut the throttle when the nose goes low...
As for guns, i personally would ditch the gondolas as they dont do your handling or climb rate any favours - neither of which you can really afford to compromise on, IMO. Take the 20mm hub cannon and thats it. The 30 is brutal but very hard to hit with if you aint used to it. The 20 is a better option as its easier to hit with. If your intercepting buffs, put the gondies on and the 30mm spud gun in the spinner, but not for fighter work.
The G14 reverts to very average when the WEP runs out, so make sure you use it wisely and know when to let it cool down.
