I love the connection between Setsuna's clumsy overzealousness at cooking and lovemaking.
(I don't think Setsuna's cooking (dis)ability has been touched on outside the SIFAS event "Come Enjoy These Special Sweets", but even without that, the way her over-enthusiasm switch gets hit when she's talking about her favorite anime or light novel or anything, really, makes it very understandable.)
I think the "magic drink" bit wasn't boosting confidence, per se, but just getting them both to relax a little — you'll notice they both drank some. Once Setsuna wasn't stressing over having a checklist to get through, and once Kasumi wasn't worried about that, they could handle things much better.
(Now that I think about it, I do sort of wonder that we all seem to take for granted that Setsuna's lover would keep using her stage name. It makes a certain amount of sense — she's much more herself as Setsuna than as Nana — but it's still a little odd.)