@DrStreetmention yes, I tend to ship back any hand without any white sources, unless it is explosive enough (Turn 1 TKS, Spheres+Wastelands or the like). About Lotus Petal, I think I'll try it next tournament instead of Mox Sapphire: if we have Stony SIlence in play it will be as bad as any mox; but again, if we have Stony Silence, it means that we had white mana already.
About mulligans, I felt that I mulliganed a lot (8 times in 17 matches, one of them down to 5, though I won these). I'm not sure it that's inherent to the deck or if I was just unlucky with my opening seven. Sadly, I didn't track which hands didn't keep.
"They vintage you". I like it
@ssasala you're right, I haven't tested so much against Shops (I sleeved the deck and take it to the tournament without any testing TBH), and I was lucky in my Top 4 matches; probably if they start explosively we will have a bad time. I'm sure we can fight back, though, as we can also cripple their manabase with strip effects and Stony Silences and play big creatures, like they do.
About Priests in the SB, that was mainly a meta call; Catalan Vintage League is an endless ocean of blue decks (Jeskai and Paradoxical builds being a 60-70% of the field), so I felt Priests were not needed maindeck. If your meta is more healthy, maindeck Priests are great for sure (though I'm not sure what would I side out to make room for them).
I haven't tested Serenity yet; I don't feel comfortable with so many non-creature White cards (against Shops post-board, I like to play only lots of mana, as much creatures as possible and Stony Silences) . But if the next matches against MUD go bad, I'll try it for sure.