Is there a viable possibility that Rashi understands that Rava holds that for a silk garment (meshi) it is first ideal to use meshi tzitzis strings and secondarily acceptable to use wool/linen strings. Saying so allows for a smooth reading of four separate Rashis on 27b in close proximity. Not saying so, leaves questions in Rashi's wording in multiple places and begs for a pshat in how Rashi reconciles Rava in entirety: the smichos referring to strings and requiring wool/linen, the tzitzis requirement in a silk garment, the 'haknaf' allusion to use silk strings on a silk beged - is that preferred or acceptable, is inserting wool/linen strings into silk garment preferred or acceptable, what lashon would one use to describe the minimum string requirement in a silk beged - at minimum silk strings or other lashon, how one would ddescribe inserting wool/linen into silk garment - 'also acceptable' or other lashon etc.
1) I do not see at the moment where it is Mashma in Rashi that l'Chatchilah silk strings are better in a silk garment, and that wool or Pishtim strings are only good b'Di'eved in a silk Beged.
Rashi (DH Min Kanaf) does write, "And if they are silk then Tzitzis of silk exempt them," but I do not think that this means that silk is better than wool and Pishtim. Silk, wool, and Pishtim can be considered as equal.
It is true that the Shulchan Aruch (Orach Chayim 15:5) writes that the standard way is to sew a garment with the same strings as the material of the garment, but this is not a Halachah but simply the practical way that people usually sew clothes.
2) I found that the Hagahos Maimoniyos (on the Rambam, Hilchos Tzitzis 3:6) writes that his teacher (the Maharam of Rotenburg) said that when the Gemara says that wool and Pishtim may be used as strings either with their own material or with other materials, this applies only if the wool and Pishtim strings are together. He writes that for this reason the custom is to use silk strings for silk garments.
I found that the Shulchan Aruch ha'Rav (9:4) writes that the Din of the Maharam of Rotenburg is derived from the verse, "... wool and Pishtim together, shall you make strings for yourself." This suggests that according to the Maharam of Rotenburg, this is a Din d'Oraisa.
According to this, we have a Shitah that maintains that mid'Oraisa there is a preference to having silk strings on silk garments. However, I do not know if there is any reason to believe that Rashi in our Sugya also agrees with the Maharam of Rotenburg.
3) I then found that there is a responsum by Rashi (in Teshuvos Rashi #84) which starts:
"A garment of linen with Tzitzis of linen is valid" (this part of the Teshuvah of Rashi is cited by Tosfos in Menachos 40a, DH Sadin, who says that the opinion of Rashi is that according to the Halachic conclusion, Kil'ayim are not permitted in Tziztis).
Rashi continues:
"'Tzitzis ha'Kanaf' -- the Tzitzis shall be on the corner. This implies that the Tzitzis should be the same material as the corner."
It seems from this that according to Rashi, the Tzitzis should always be the same material as the garment. This would include a silk garment for which the Tzitzis should be silk.
4) We find that Rashi's Shitah is explained at greater length by Rabeinu Shlomo in Shibulei ha'Leket (Mirski edition), in Hilchos Tzitzis (pp. 76-77). He points out that it would seem more logical if the verse in the Torah would have stated that one places on "the corner the Tzitzis." That is, we would have expected that the word "Kanaf" should come before the world "Tzitzis." Why did the Torah say it the other way around, putting the word "Tzitzis" before the word "ha'Kanaf"?
Rashi answers that this hints to us that the Tzitzis should be the same material as the corner. Rashi continues and asks how do we know that if he used Pishtim strings, he is Yotzei? He then cites the Derashah mentioned in our Gemara that wool and linen are effective for any material.
5) So it does seem from the above that according to Rashi, the first choice is silk strings for a silk garment, while wool or Pishtim strings for a silk Beged are only a second choice.
1) I would like to go back to Rashi on 27b and summarize what we found.
Rashi (Shabbos 27b, DH Min Kanaf) writes: "The Tzitzis should be the same material as the Kanaf. If the garment is silk, a string of silk exempts it." The next Rashi cites the verse, "wool and linen," and Rashi writes that this fits with the verse, "You shall make strings for yourself." He explains that this implies that you can also make strings from wool and linen (Pishtim).
2) There is a suggestion here that the first choice for a silk garment is silk strings, while wool and Pishtim strings are only second choice. Merely on the basis of the Rashi in Maseches Shabbos it would be rather tenuous to come to this conclusion, but Rashi cited by the Shibulei ha'Leket strengthens the possibilty.
3) The Shibulei ha'Leket (by the way, I see that the Hilchos Tzitzis by Rashi appears in the older standard edtions of Shibulei ha'Leket on page 378) writes in the name of Rashi that the verse hints that the Tzitzis should be the same material as the Kanaf. Then Rashi continues, "and how do we know that if you put on Pishtim strings that you are Yotzei?" Rashi then cites a verse. There is an implication that with Pishtim strings, one is Yotzei only b'Di'eved since the Torah wants, l'Chatchilah, that you should put on strings of the same material as the garment.
According to, this we have support for your original suggestion, Daniel, that ideally for a silk garment you should use silk strings.
Pesach Kasher v'Same'ach,
Dovid Bloom