Haven’t had a chance to read the whole thread, but agree very much with Oomis and SJSinNYC, here. A few points:
1) Look up tshuvah in Reb Moshe about a yid who came from Poland, and asked whether changing from the long levush of his hometown to the shorter levush of the USA would be considered dressing in a non-Jewish fashion. Reb Moshe replied, no, it is changing from one Jewish mode of dress to another Jewish mode of dress, and he is allowed to wear the shorter jackets, since many fine Jews wear shorter jackets.
2) The core of this debate is about the nature of frumkeit in general. Is frumkeit to separate from other Jews, and think I am better than those guys, or to inspire other Jews. If the former, than wear a type of dress which although it has nothing to do with yiddishkeit, it is obviously much different than everybody elses. This will cause me to keep my distance from them, and they will certainly do the same to me, since they think I look totally weird. They will not want to learn more about frumkeit, since they have absolutely no interest in becoming part of any group that dresses in such a totally weird way that makes them look like a public spectacle. They would also not want to put on a pair of Groucho Marx plastic eyeglasses, nose and mustache things in public on a day to day basis, and would politely decline an offer to join any religion which had that as a requirement for exactly the same reason. Please understand that this does not make them bad people. It is just that most people do not have the guts to make themselves look like freaks in the eyes of the rest of humanity.
3) Note similarly that for the purpose of kiruv, if in the interest of full-discosure you told a non-frum professional with advanced degress who attended a reform synagogue his whole life and loved the state of Israel where he marched in the parade with his community every year, “Come, let me introduce you to authentic Judaism. I will teach you to hate the state of Israel, and also that after 13 years old you are not allowed to learn secular studies. That even includes no high school algebra, geometry, trig, and certainly no college. Doesn’t that sound like a great way of life. Come join us to learn more.” I think you will not get that many takers, and some may disgorge what they have just eaten. (The other alternative is to be dishonest and try to hide the anti-israel and anti-limudei chol chareidi attitude from them while you work on being mekarev them, and only at the end spring it on them, or let others spring it on them by surprise. Seems like dirty business practice to me.)
However, if the goal of yiddishkeit is to inspire people towards ahavas chesed and ahavas yisroel and ahavas habrios and ahavas haborei, and the simcha of learning a blatt gemara, then the first thing is not to alienate others or make them feel uncomfortable in any way. Make them see that frum people are the pinnacle of what humanity can be. Make them see that whatever their background they can enjoy life more with Torah. It is all darchei noam. Make them feel that they can continue to dress like a well-dressed professsional wearing nice colored suits and ties. They do not need to wear a hat, since it has no connection to Judaism. Only a small stylish yarmulka of whatever type or color or material they prefer, to remember the Creator. (And if that will cause them financial hardship then Reb Moshe says even a yarmulka is not necessary.) Do not put up needless barriers with our beloved brethren. Show them we are just like you and appreciate everything you do and your talents, but the Torah can give your life a bit more spice and happiness. Be meurav im habrios, not put up a wall, which is exactly what a black hat is.
4) Regarding those who say that looking nice is a breach of tznius, I have the follwoing question: Is a girl with crooked teeth not allowed to get orthodonture since that might make her super attractive with a perfect set of pearly white teeth that light up a crowd. Does anybody know of any poskim who hold that way? From here I conclude that tznius has nothing to do with being attractive, whatsoever.