Take off with bombs, drop them as soon as you are at a safe alt, then land and taxi onto the rearm pad. Wait 30 seconds and when you get rearmed, you'll notice the landing gear compress from the weight of the bombs.
Also, remember that the operational weight of the P-38 was over 16,000 lbs., over 18,000 lbs. with 2k of bombs. So, when you drop your bombs, the weight only decreases by about 10 percent. A change that you'll notice, but maybe not as dramatically as you expect.