Well as someone recently pointed out in another thread the Samsung 30T is available at
https://voltaplex.com/samsung-30t-21700-battery-inr21700-30t (only in bulk it seems). But there aren't that many 21700 tube mechs out there yet, and AFAIK with the XXX/11:11 the 21700 batteries just won't fit so another option to maybe consider instead would be the Vaperz Cloud Sarov mod.
The Comp-S switch is essentially an improvement over their older VC Tech switch, as there's no arcing and no misfires with the Comp-S, whereas, if you use it with 2 batteries in series configuration, the VC Tech does arc, and eventually it starts to misfire as a result from that. The brass version of the XXX uses the VC Tech, but I also bought the separately sold brass bottom section for it, and that uses the Comp-S. AFAIK the brass XXX was the last mod that they released with a VC Tech, and, at the same time also, it was world's first 20700 tube mod... AFAIK all the later versions/variants (copper, white brass, stainless, black, anodized aluminum blue or purple or red, 11:11, UK LE, Sarov) all use the Comp-S.
Whether or not you need the extension piece is entirely for you to decide... series mechs are a very different style of both coil building and vaping. I like both styles, but not everyone likes what I like. With 2 iJoy 5-legged 20700 batteries in series, I can vape pretty much all day without me having to change out the batteries... this is a LOT longer than with 2 Sony VTC5A batteries in series. In single battery configuration, this difference is not that huge, but nevertheless, it's there─albeit with a single battery you can argue that you still prefer the VTC5A because it initially hits harder than any 20700 available to us, which, as a matter of true fact, it does, BUT... like I said up thread, it doesn't take very long until the 5-legged iJoy 20700 catches up.