The thing about "tariffs" in Vic2 is that, as others have already pointed out, POPs only face one price when purchasing goods. Your POPs NEVER have to choose between a domestic automobile or a foreign one. If a domestic one is available, they buy it first. Period. (Bear in mind your SOI is considered a domestic market.) If not, then they buy the foreign one
whether the price is increased or decreased by a tariff. The only time the tariff makes a difference in purchasing behavior is when a POP buys something that has a tariff and then runs out of money to buy the next time. Setting high tariffs can hurt POP purchasing power if your country imports POP goods. If not, then you can't raise any significant revenue via tariffs anyway.
Ditto for factories.
This is important, because in the real world, tariffs often create a Wal-Mart moment. You see two pairs of shoes. One is domestic and costs $50. One is foreign and costs $100, thanks to a tariff. You favor the domestic product because of the price difference.
But in Vic2, POPs never make that choice. They only ever face one price at a time; and that price is decided by the WM and whether a tariff is in place or not, and whether the good is domestic or not. You POPs and factories never "shop around."
What this means is that tariffs more or less act like a sales tax compared to the income tax natures of other taxes. This means that tariffs hurt any industry that depends on any imports, but it doesn't help any industry that must compete with foreign made goods.
Clever players in this thread have already figured out that subsidizing imports can make factories more profitable, helping the economy in certain ways. I will only add that because banks never loan to capitalists or aristocrats, and because capitalists and aristocrats get so wealthy that they start filling the national banks with huge cash reserves that never make it out into the economy again, taxing the rich and subsidizing imports is the magic tool that can help the late game economy grow. High taxes on the rich (high enough that they still build factories and buy all luxury goods) can be profitably used to cut down on the costs of factories that import certain goods.
Helping the middle class and poor promote is just a handy side effect that helps everyone.