Would the higher CCA yuasa "GYZ" batteries, generally speaking, have equivalent or shorter expected service life than the "standard" yuasa batteries?
It's possible some may have shortened lifespans, depending on how they got those extra amps. The exterior dimensions to fit a Harley are pretty much fixed, so getting more CCA out of the same overall size package involves finding ways to add more plates. That could entail thinner plates and separators, among other changes. So far I haven't heard or seen reports of early failures, but the sample size of people using these is probably pretty small. I dare say most Harley folks haven't heard of this battery yet.
I'm also interested in seeing some reports from folks running lithium batteries. I know there are a few on this site running one of the various units being offered, and one person has given a positive review so far. One of my issues with LI batteries is something anyone using LI powered power tools is aware of. First, performance doesn't taper off like lead acid batteries, but just suddenly shuts down. Second, LI batteries don't do well in cold weather. And of course third, there are too many safety issues with poorly made LI batteries, as seen with all the fires in laptops, phones, hover boards, aircraft (Boeing), etc. There are too many flaky sellers of Chinese batteries packaged to fit a motorcycle, and I'd highly recommend anyone looking for LI make certain they are getting batteries from a reputable source.
JMHO - Jerry