Wanks back then was still weak enough to be scarred by fruitless BB while he later created a gap so big that BB couldn't fully close than it even after getting two of the strongest fruits in the verse. It's not likely but it's possible Saint Shanks is stronger than Shamrock although it makes more sense for Shamrock to be stronger, if their potential is equal, as Saint Shanks lost an arm.
tbh we still dont know how exactly bb was able to scar shanks , his strength is still mysterious to this day.
also shanks is still vary of blackbeard ,
that wouldnt make sense if he was a weakling during the days he got scarred , he is in his youth just like luffy.
the shamrock and shanks stuff is pretty intense tho , was shanks hiding his strength ?
or is shamrock just that strong.
his potential is up there tho.