At this point in GTA V's life, do you think a significant amount of customers are uninformed enough about the product to consider the Steam review score (but not check the actual reviews) before making their purchase?
Will those reviews dissuade people that were interested in buying it due to modding, particularly due to Youtube videos by personalities? Yes, and YT is one avenue where a lot of noise needs to be made. Will it dissuade people that are buying it for GTA Online? No, and those potential whales are what the publisher and developer care about right now, enough to kick the mod hornets nest.
Yes? Rather than just spraying shit all up the walls actually have an angle of attack, if you want to organise a protest then make the action about not playing GTA Online and not buying any Shark Cards until they reverse this decision. If their userbase or revenue drops by any noticeable amount, then you've got their attention.
By actually telling them? Jesus, you've got voices and you're using them currently, direct them in a way that will have an impact. Tweet, e-mail, set up a Reddit post, say why you're not happy with that decision and what your action will be. Hell, get some big GTA mods Youtube personality to spread the word. If they see 30k properly worded e-mails (not just FK U ROKSTAH WERE MY MODZ?), if they see their GTAO playerbase drop 20%, if they see their shark card revenue drop 5% they will notice and act.
Noise works when directed correctly, don't waste your opportunity by being a rabble that the company decides is worth ignoring the ire of.