There should be no necessity to go higher. Just the ratio should be the same as your screens ratio.
Sorry it's hard to asses your problem from so far away
What you can try is to try out different resolutions of differen ratios until you find the one that fit's your screen better. But it does not seem to be the problem if your run the same resolution with IL2 and it is fine there.
Are you sure that IL2 (I never played it) does not stretch the picture?
I think your best call is to play with the resolutions and to see how behaves...maybe gives you a clue to fix your problem. But I understand that can be annoying beacause you alwas have to restart SH.
Also you could try windowed mode. Then you might see better if your resolution needs to be higher in the vertical or horizontal.