I don't know if I would use as emotionally loaded word as "sinful", but I do believe it is prohibited. The Rambam talks pretty harshly about the person who uses the Torah for making a living. Hilkhos Talmud Torah 3:10:
כָּל הַמֵּשִׂים עַל לִבּוֹ שֶׁיַּעֲסֹק בַּתּוֹרָה וְלֹא יַעֲשֶׂה מְלָאכָה וְיִתְפַּרְנֵס מִן הַצְּדָקָה הֲרֵי זֶה חִלֵּל אֶת הַשֵּׁם וּבִזָּה אֶת הַתּוֹרָה וְכִבָּה מֵאוֹר הַדָּת וְגָרַם רָעָה לְעַצְמוֹ וְנָטַל חַיָּיו מִן הָעוֹלָם הַבָּא. לְפִי שֶׁאָסוּר לֵהָנוֹת מִדִּבְרֵי תּוֹרָה בָּעוֹלָם הַזֶּה. אָמְרוּ חֲכָמִים (משנה אבות ד ה) "כָּל הַנֶּהֱנֶה מִדִּבְרֵי תּוֹרָה נָטַל חַיָּיו מִן הָעוֹלָם". וְעוֹד צִוּוּ וְאָמְרוּ (משנה אבות ד ה) "אַל תַּעֲשֵׂם עֲטָרָה לְהִתְגַּדֵּל בָּהֶן וְלֹא קַרְדֹּם לַחְפֹּר בָּהֶן". וְעוֹד צִוּוּ וְאָמְרוּ (משנה אבות א י) "אֱהֹב אֶת הַמְּלָאכָה וּשְׂנָא אֶת הָרַבָּנוּת", (משנה אבות ב ב) "וְכָל תּוֹרָה שֶׁאֵין עִמָּהּ מְלָאכָה סוֹפָהּ בְּטֵלָה וְגוֹרֶרֶת עָוֹן". וְסוֹף אָדָם זֶה שֶׁיְּהֵא מְלַסְטֵם אֶת הַבְּרִיּוֹת:
Nevertheless, whosoever sets his heart to pursue the study of the Torah but do no secular work at all, and permits himself to be supported by charity, behold him, he blasphemed the Name, and degraded the Torah, and shadowed the light of religion, and caused evil to be brought upon himself, and deprived his own life from its share in the world to come; because it is forbidden to enjoy aught in this world in return of the study of the words of the Torah. The sages said: "Whosoever enjoys aught in return of the study of the words of the Torah takes his own life away from the world" (Pirke Abot, 4.7). They have, moreover, commanded and said: "Do not make them a garland by which to be considered great nor a qardom lachapor bahen -- a spade to dig with them." (Ibid.) Again they have commanded and said: "Love manual labor and hate rank". (Pirke Abot, 1.10); whosoever studies the Torah and does not acquire at the same time a manual trade his knowledge of the Torah will be nullified and bring about sin" (Ibid. 2.2.). The end of this will be that he will rob people for his living.
(Notice the Rambam quotes the same mishnah as did the question.)
The reasoning of Pirqei Avos and the Rambam would seem to apply to someone who uses Torah for business advice.
Someone who supports Kollel may say we simply do not hold like this Rambam. In our culture, getting paid for learning is indeed common (from YU to Ponovezh, after all). And there are various approaches to this question in which we do both support the idea of getting paid for studying in kollel and hold like this Rambam.
To my mind, the simplest resolution is that there is a difference between someone who learns in order make a living, akin to someone with a skill in programming computers making his living doing that; and someone who wants to learn, and therefore takes a paycheck to make it possible. This is how I understand the leniency of being allowed to pay someone "sekhar batala -- pay from being idle [from a job]." Turning Torah into just another job is problematic. Kollel isn't that, though.
And I would see the same distinction here. The Ramchal is telling you how to be holy, which includes all of life, including business. In practice, the holy way of doing things "just happens to be" pragmatically superior and more successful more often in the long run. But if we take the Ramchal's advice and use it for that practical success, you took his words of Torah and made them a "qardom lachapor bahen -- spade to dig with them".