So, have now extensively and systematically tested.
With original aerial image, I come with the PMDG 737 to about 42 FPS. With disabled aerial image on about 18 FPS with the PMDG 737. With "Overflow" aerial image, the frames are even worse than the original aerial image (22FPS)! And the "Overflow" aerial image is totally pixelated and the colors are not right (bad color management?). So I would advise against all.
I stick to the original aerial image and recommend it to everyone else.
PS: My system
i7 2600k 4.2Ghz, 32GB RAM GTX980TI