Advantages of converts (in spiritual terms)

Question

I've heard before that the Ari Zal says the neshemas ("souls") of gerim (converts) comes from the highest places. Is there any source for this? Where is this brought down?

As well what other "spiritual advantages" do we find about gerim in any Jewish writings?

(My reason for asking is because an elderly geiorus (woman convert) was suggested as a match for an elderly man who recently lost his wife. The brother of the man literally cringed at the idea that his brother should be matched with a convert. This woman happens to be Jewish now for more than 20 years and, unfortunately, never got married. She is very frum and highly respected in her community and among her friends which include very important Rebbetzin's around the world. I would like to stress that if the only point he doesn't like is the fact that she's a convert then he should know what it says in seforim (whichever they might be), the advantages in spiritual terms of converts.)

Answer 1

There are these statements, from Kehillas Yaakov, s.v. גרים:

גרים הן מזיווג הנשמות בג״ע זוהר שלח...וגס מזיווג הצדיקים בחייהן אותו זיווג שאינו מוליד ממנו וולד אעפ״כ אינו לבטלה שנברא ממנו נשמת גרים

"Gerim come from the union of souls in Gan Eden (Zohar, Shelach)...and also from the marital unions of tzaddikim during their lifetimes: their unions that don't engender children are nevertheless not wasted, as the souls of gerim are created from this."

and

וז״ש מרע״ה ליתרו אל נא תעזוב אותנו כי ע״כ ידעת חנותינו במדבר והיית לנו לעינים רומז לו איך חביבים גרי צדק לפניו ית״ב וגדול בעיניו נצוץ היוצא מעמקי הקליפות והראי׳ לזה שהרי טלטל למרע״ה עס ש״ר מישראל והזקנים גם כביכול שכינת עוזו הארון והמשכן והוצרך לעשות נסים עפ״י שינוי הטבע בהורדת המן והבאר והעננים כ״ז עבור שיוציאו נ״ק א״כ נראה בעליל איך חביב לפניו נצוץ קטן שמלובש עדיין בעפריות בדומם כ״ש בנצוץ גדול ורב כמוך אתה חביב עלינו כגלגל עינינו

"Thus Moshe says to Yisro, 'Please do not leave us, since it is because of this that you know our encampments in the desert, and you will be our eyes.' He is hinting to him how precious genuine converts are to Hashem, and how great in His estimation is a spark [of holiness] that comes from the depths of the kelipos. The proof: He moved around Moshe, along with six hundred thousand Jews and the elders, plus - so to speak - His Shechinah, the aron and the Mishkan; and He had to perform miracles against the natural order, by bringing down the manna, the well and the clouds - all so that they could extract sparks of holiness. We see clearly, then, how precious to G-d is even a tiny spark that is still enclothed in the inanimate dust. How much more so, then, [says Moshe to Yisro,] that a great and important spark like you is as dear to us as our own eyeball!

Further along, he notes several sources that gerim relate to the "feet," as contrasted with born Jews who are the "hands" - but then, healthy eyesight (i.e., the leaders of the community, its "eyes") depends on the feet (see Berachos 43b, bottom).