The only design difference is the HD sticker on it.
We can always think that, but without proof I don't think I'd state it as fact.
If you look on the website for the Deka batteries they have the same disclaimer next to the ETX30L as was reported by Chevy in his post. And if you look at the actual specifications page for that battery, you will note that it is rated at 26 amp-hours versus the 28 amp-hour rating for the stock Harley battery. The Deka is rated at 400 CCA, which is a much more important rating than amp-hours, but it's pretty hard to directly compare that spec to the Harley battery since Harley doesn't list CCA in it's catalog. Last time I saw a CCA rating for the Touring battery from Harley was in 2008, and it was 365 CCA. The latest batteries may be higher, I have no way of easily determining that.
Just because the same supplier makes two different branded batteries of the same dimensions doesn't necessarily mean the batteries are identical under the labels. Case in point, go visit one of the big automotive battery supplier plants and watch Diehards and Motorcrafts and various store brands all go by on the same line. But within the exact same sizes you will find wide disparity in performance and pricing based on what's actually inside those same size cases. There are higher and lower cost plate and separator materials, different vibration absorbing materials, more or less dense packaging (number and thickness of plates for instance), different valve construction on batteries so equipped, etc.
Anyway, considering the huge difference in retail price, even
if the ETX30L isn't quite as "good" as the OEM battery spec wise, who cares? Obviously there are a ton of folks out there using them in their Harley's who are completely satisfied, so they must be good enough.
Jerry