I conduct services as a chazzan in an Ashkenazic shul, and as such, I do everything, even my own personal Amidah, in that nusach (which I happen to personally enjoy very much). An interesting thing about our shul is this: our rabbi, a talmid of Rav Soloveitchik, zt”l, told me that on Yom Kippur, I should say the Avodah, which is said silently, in Sefard, since the Rav preferred this. Also, we open Neilah with El Nora Alilah. I asked him why the shul includes this very Sephardic piyut, and he said that because the shul used the Silverman machzor many decades ago, and this machzor includes this very Sephardic piyut for Ashkenazim, it became beloved to the congregation over time, and the minhag stuck. Interestingly, I find that our Ashkenazic rabbi mandates that many piyutim be skipped over in the high holidays davening, and no piyutim are inserted into the Amidah during the year at all, although I don’t know how many Ashkenazic places nowadays still include the Krovetz l’ Purim and whatnot).
While I use Sephardic havarah when I speak about torah and whatnot, I do daven (for the amud) in Ashkenazic havarah. I just feel that if I’ve taken the time to learn proper Ashkenaz nusach hatefillah, than I should pronounce the prayers in the Ashkenazic manner, and I also feel that from a musical and phraseology perspective, the nusach “flows” more smoothly and just works better when Ashkenaz havarah is used.
Regarding “suf” and tuf”: I heard a story about a kid who was personally very frum/yeshivishe-style (and involved in NCSY) but whose parents sent him to a coed community day school that was very Modern. Whenever the kid would read out loud, the teacher would correct his sufs with tufs (i.e. Shabbat, not Shabbos). The kid one day said to his teacher out loud, “I’ll tay it how I want to tay it.”