Excin,
The same problem exists in the A320s. The problem is that the A320 can't get has high as the 737s. A charter 320 will never see 390, especially tankering. The 737s cruise higher. IF you have a problem with fuel icing, simply fly the aircraft down around 350 like an A320 and the difference will be enough.
I flew a corporate 320 for a while and we would tanker and with only 50 seats always went to 390. Landed and iced up. pretty common.
I submit that icing has more to do with cruise alt than wing design.
Cheers
Wino