Yes, you do. That, and buy a talent that increases the cap. There's a cap not on actual level, but on numbers: if the boss has more HP than the cap, you simply won't do any damage. GBG's 11th objective increases the cap (by a lot), and there is also a talent in the Utility Tree that does that (Arithmagician, lvl 6, but be warned, it is VERY expensive). With those two (Arithmagician is at lvl 10 for me, I have 158B idols) my cap is 3665. And yes, in some formations (especially RP2) I can reach it easily.

At the point you're at, GBG is definitely more than enough, you won't reach the new cap for quite a while.

PS: I don't want to crush your spirit, but Arithmagician lvl 11 costs 47.1 B idols :D Luckily, you won't have to worry about that for a long time.

Yes, one rank of Arith might be worth its cost (500M idols for a factor of 1e20), but GBG Tier 3 11th (To Infinity and Beyond) only costs the time to run the prerequisites and gives a factor of 1e92 (from 1e308 to 1e400).

Then there is RPT Tier 3 11th (Bullet Hell) that gives a factor of 1e200 (from 1e400 to 1e600). At this time, RPT 3.11 is the single biggest increase in maximum damage.

FWIW, based on my experience and calculations, the maximum areas and maximum boss bonus idols (with all Tier 2 11ths but before any Big Earner bonus) would be:

308: 1,585 ------ 560,115
328: 1,685 ------ 834,687
400: 2,065 ---- 3,793,092
420: 2,165 ---- 5,648,142
600: 3,095 -- 228,860,511
620: 3,200 -- 347,591,814

Also from my notes, it is almost certain to get at least 30% of these boss bonus idols and almost never more than 50% of them on a given run.

Average - 39.82%
Median -- 39.71%
Maximum - 53.54%
Minimum - 22.32%
Resets ---- 466

Then also, regarding the "as they hit %50 bonus" comment, the only "passage of time" recharging of the bonus is 5% once per 24 hours. The other recharge is 1% per 250 areas completed on Free Play objectives. So, when you do get to the point of only playing Free Play objectives, my advice would be to pick the best campaigns' Free Plays to go deep and then the others play for 30 minutes or for as much Sprint Mode as you have and then reset. (This also makes MaM 3.11, From the Ashes, quite useful for the doubling of Sprint Mode which gives Sprint to area 1,000 so 4% from a single sprint.)