It's hard to say. A lot of Marines were sunk by Luffy's burst of Haoshoku, I could see a lot more being dropped by someone like Shanks. The size of the crew makes no difference, as the crew of as Yonkou is going to be mad strong. Losing Beckman would be too much for his crew to win in Marineford though, I would say. If he were to go to rescue Luffy for example, I think he'd manage it.
It was not the fodder that decided the outcome of the war, but the big shots. WB pirates vs Marines was all about who's gonna bring down the leaders of each camp. Now imagine for example if Sengoku, Garp or the three admirals were out of the game by some miracle, then the WB pirates would've won the war. The moment Squardo injured WB, the marines already won the war. Old as he is, and injured, it was a miracle that WB managed to do such damage.
So, if Shanks were to be at the place of WB, I think that he may save Ben Beckman, since he's still younger, and would be more efficient that WB was, that is if his crew will manage to hold down the admirals for enough time, as much as I love Shanks, if the admirals keep doing whatever they want in the battlefield like in the war on summit, he won't win no matter how hard he tries.
But let's suppose for an instant that the war to save Beckman takes after Garp and Sengoku left the Marines, and that there's no stronger than the admirals. Then it is possible for Shanks to actually win this war. He'll defeat the admirals with the help of his crew somehow, but then, nobody will be able to stop him anymore, and the moral of soldiers will drop. And we know that moral has it's own importance, as seen when WB and Ace died, the guys were really motivated, especially Akainu who was eager to finish off the WB pirates.
The key to this war is how to stop the admirals, and possibly the Shichibukai (in the war, they didn't seem to be really interested by the conflict anyway). And to stop one of the three big forces in the world, you gotta be really smart and strong.
