This is where I throw in my "it depends" comment.
It is not the most efficient gph strategy for sure, but depending on your playing style it may be the most efficient for you.
Some people can't sit down for two hours and play continuously and prefer to build a slow army for farming. If that's your situation then I think Balloons are awesome for that. (I personally love balloons/minions btw)
I've spent a few days in Masters III and Crystal I as both TH9 and TH10 trying to farm gold with balloons/minions and even boosted I can attack roughly 4 times per hour (maybe 5) so if I rocked 500k gold every single base I'm at best going to do 2M - 2.5M per hour (boosted) with fairly long pauses in between. Obviously 500k gold per attack isn't very practical and when you have that oops failure it really hurts.
The other factor for higher level farming is the defensive losses (especially as a TH9) are much higher. Leaving your TH outside in Masters III is a death wish because you're guaranteeing the guy a victory, so you'll get rocked for max loot almost every time you log off which has to factor into your overall gph stream.
So, from a gph "efficiency" standpoint balloons in masters is not the fastest way to get gold, but if it fits your playing style and you recognize that, there's nothing in the world wrong with doing it.
Weekend before last I got bored, so I jumped up to Masters just to play with my ballons for the weekend. I didn't get any great loot per se', but I sure had a lot of fun.

I was upgrading all 4 of my AD's (i know, bad idea) so I had some fun times on defense too. lol