I doubt it'll affect it at all. I think it will make his HS/Slam more predictable.
My understanding of basic mob scripting is that they have a simple "AI" that gives each of their abilities a certain % of happening if it's cooled down, and those abilities have cooldowns. Abilities that happen like clockwork have 100% or near-100% priority. For example, Magmadar will always Panic if he can, but it has a 30sec cooldown, so he pretty much does it exactly every 30sec. Onyxia's Bellowing Roar, however, is a lower % priority, so while it will NEVER happen twice in 10 seconds (due to the cooldown), it sometimes will take a while after it's cooled down before she does it.
My guess is that Gruul was favoring Cave In a fair bit, which led to odd timing delays in abilities like Slam at times -- Slam would be cooled down, but every time his AI randomized its next step, it would keep "rolling" Cave In and so the next Slam would be severely delayed. They don't want Slam to be on a clockwork timer or they'd just make it 100% priority (but then people could reliably hide behind rocks with no downside and it'd change the fight), so they're lowering the Cave In % so that it's much less likely he'll decide to keep doing Cave In for 30sec straight instead of other abilities.
That is my largely-guesswork prediction based on what I have observed of mob behavior over the past couple of years: He'll use his other abilities more predictably, and not much else will really change.