I got the EarthX battery (ETX18B) today and installed it. I got it from motolithium.com (herrhelmet in Vendors) for about $200 shipped. It is a snug fit, but works fine. I snapped some photos if anybody is interested.
The first several pics show the size difference. The new battery only weighs about 2 pounds.
It's smaller in dimensions, other than height (see below). This makes for a snug fit in the battery box, but you don't really have to force it. I still used the rubber battery tie down, but it really isn't necessary. You need to route it over the terminal rather than over the top due to the snug fit. You can also see the foam supplied with the battery should you need to secure it further. I only used a couple.
Here's a pic of the current Yuasa in the battery box (excuse the rat's nest of extraneous wires):
Here's how the EarthX battery fit:
And the final fit, with some of my rat's nest tucked into the new found space:
At this point, I can only say it fits and starts the engine. I can't really endorse it until it lasts several years. But if it truly is similar to an 18Ah battery, I'll feel a little better using my heated gear and lights during my cold commutes. This type of battery should tolerate a partial draining better. I'm thinking I can feel a bit more comfortable if I see the battery going down to 12.0 volts, knowing I'm probably not hurting the battery. I'll still be watching that voltmeter. Time will tell.