Cantoresq: I’m mostly with Joseph and Feivel on this one.
I think it’s very admirable that you are capable of preparing a talmud shiur for the mispalilim of your shul.
But if it was me attending your shiur, because it’s Shabbos, it would be a different learning if your mode of dress was perceived to reflect proper respect both to the learning and those attending the shiur and of course to the holy Shabbos.
Shabbos should be accorded at least the amount of respect for dress as your weekly business attire. What kind of message would a gifted attorney send the court if he argued his case in casual dress?
How you “relax” b’tzena, in the privacy of your home, is one thing, but I don’t believe that your relaxation should be extended to a “mikdash m’at”, your shul.
Why do you wear a suit in the morning but not the rest of Shabbos, it’s no less special? How would you or the others feel if your Rav dressed in this fashion, especially in shul?