No, it wouldn't be targeted at your average skiller, more at the people with money to burn going for 120/200M Herblore or whatever who will throw cash at it. I suppose it would also be helpful to people who could save up say 50M at once and go on an Herblore training spree. Because if you think about it, there are very few actual money sinks in the game. Prayer, Construction, and Herblore are not even true money sinks for the most part. For example when you train Prayer, while you do buy bones, do the coins you spend disappear? Nope. They go to whoever sold you those bones. That might be another player who spent some time slaying dragons for money (props to them) or it might be from a bot dumping materials after doing the same thing. And as I've pointed out years before and is further made evident by the spammers I get in the GE and in my PM, someone is buying that gold back through RWT. So, it doesn't really suck any money out of the economy, it just "rebalances" (or changes hands might be a better way of putting it) it in the same way that money through bonds sans tax do.
Also RuneScape has had a sort of logistically correlated benefit to cost ratio for a while. Look at Prayer again. You have a whole huge table of bones you can use at an altar, from ridiculously slow (accursed ashes) all the way through Frost Dragon Bones or Searing ashes. Or take Smithing. You have Artisan's Workshop which has a huge range of options in terms of XP rates and hence efficiency, and much to my surprise, I have found some people that will chug vast amounts of resources straight in.
Besides the notion of skillers training Herblore seems somewhat absurd. The major potions (super-antifire, prayer renewal, overloads, etc etc) are either combat related, untradeable, or both. Granted the juju potions might do something, but obviously a skiller isn't go to say '85 Herblore QBD here I come!' If you're referring to skillers making profit, well true I suppose, it would be nice for them to be able to use a high skill to mix Herblore potions and sell them off at a profit, but until those potions are slightly less marginalized I don't see that happening or a slight XP buff changing much of that. I can see where you are coming from though, and yes unfortunately skillers do get shafted much of the time - that's a story for another day.
But if you are really suspect of me coming after people's skilling outfits, I guess that leaves one other option, which would be to boost them further (groan). Let them be "imbued" or something by throwing millions of GP directly from your coin pouch into your skilling outfit, which is then gone from the game, and gives a finite amount of time for a limited time boost to the outfit efficacy. Maybe 2M to temporarily boost a full skilling set to 7.5% instead of 5%, 20M for 10%, 200M for 12%. Some people will be that impatient or race to get them done in record time. I just think an idea to "fix" the economy/top-heaviness may involve using people's patience against them, so to speak.