Stop by my place sometime, I'll prove it to you.... I keep excellent cables on most everything I own. On my 966, I don't even have the battery covers, because I want to notice & clean the corrosion asap.
You know guys, the reason the 12V parallel setup cranks better is because...
YOU REPLACED THE BATTERY CABLES!
When you just replace the 6V batteries, you never touch the old nasty cables, and you wonder why it doesn't start for beans.
In order to go to the 12V parallel setup, you needed to add two new battery cables, at minimum. Plus you cleaned up the connections while you were doing it.
On top of all that, if you went direct to the starter with each battery, you've DOUBLED the amount of wire carrying current to the starter. Most of the old 6V setups have too small of a cable to begin with, and half the time they were replaced with even smaller cables.
If you put brand new double-aught cables on a 6V series setup, betcha it'll crank just as well as parallel 12's.
1/0 cables are what you'll get from CIH for the factory 2-6volt setup....