RE: iS133 vs iS122
I know you looking for a Starboard Team response here, but let me say this. I use the 50cm Mirage with an old 1998 Mike's Lab course slalom that is only 61cms wide, so I would seriously doubt that you would have a problem with a larger than recommended fin in the iS122 (if you could find any with a standard Tuttle base).
Of course, the focus would be the lightest of winds when using such large fins. The one thing I've noticed regarding the preferences of the majority of folks here on the forum is that they seem to fin for powered conditions and not the lightest winds, hence the conclusion that large fins aren't as fast or as workable. Frankly, if I use my biggest sail (8.3), I'm targeting wind averages in the sub to near mid teens, and I find that a powerful fin just offers easier early planing and superior windward performance in those conditions.