The problem isnt with ganking with capitals, it will be that Capitals will be immune to ganks. I had this discussion with Beasts of Revelations here(CCPlease Reimagine Capships - #26 by Solonius_Rex) but essentially, it boils down to this.
Supers already have such high EHP and defensive capabilities, that they will be near-impossible, or atleast not feasable, to gank in highsec. Even fully tanked jump freighters dont get ganked a lot, because of the high amount of characters needed to take down these ships.
The whole point of EVE is that you are never safe, as long as you are undocked. And allowing capitals is antithetical to this core aspect of EVE. Decreasing the EHP, or increasing the cost of Capitals and supers, will not change this, unless the EHP of a super is so drastically decreased to the point that they lose all meaning as supers and titans, which essentially means you are breaking the game.
The only way to balance this, is if Captials that are in Hisec, are constantly being targetted by Faction Police, and/or are in a permanent suspect timer status. Im sure you wouldnt agree to this, though.