The nature of shotguns is such that damage should be entirely dependent on the rounds used and not the shotgun itself (except for chokes). Because of the pressures produced by shotshells, the shot/slug reaches maximum velocity very quickly, and barrel length does not seriously affect it. Also, spread is dependent on the choke (or lack thereof) and not on barrel length. Basically, while the SG charts are fairly good, I'm not sure why, for example, a Mossberg 590 firing the same shells as a SPAS 12 is so much worse.
If you want to be consistent with WL: Firearms, calculate the ME # of a shotgun shell loaded with a slug weighing the same as the total shot weight. Subtract one to get the base SG #, then apply modifications based on shot size.
Examples:
Federal PFC157 12 ga. 3" 12x 00 buck 1325 fps
1 1/2 oz, 2557.2 ft-lbf, ME #14, SG #13
Since it is buckshot it needs no SG # adjustment (except for Huge).
Federal SC176 12 ga. 2 3/4" 1 oz #8 1350 fps
1768.4 ft-lbf, ME #10, SG #9
Since it is #8 shot it needs to be adjusted, for example to SG #2 for man-sized targets.
InstellArms flechette shell 12 ga. 3" 20g of 40 flechettes 1970 fps
2659.1 ft-lbf, ME #14, SG #13
I have no idea how to adjust this. in WL:F the only flechette rounds were SG #20. Probably, since flechette rounds are for piercing armor, I'd allow them shrapnel crits even against armor, then apply the #5-1 shot size modification based on their weight.