HGS stands for Heavy Gauge Strings. Basically, on every HGS guitar, the bridge is moved back 3mm, along with the bridge pickup route. This allows for better intonation at lower tunings. Stock, these guitars come tuned to B with 10-52 gauge strings. To be honest, the 3mm isn't that noticeable when playing it, but that extra bit really aids in intonating the guitar at lower tunings. The HGS also comes in either black or oiled walnut. Walnut bodies aren't usually found in the guitar world, and I feel it's an underutilized wood. This model has upgraded electronics, a proper three-way switch and your choice of either ebony or maple fretboards. My personal model is an oiled walnut body with a maple fretboard.
Aside from a few things, this guitar is still the same famous Horus that Caparison manufacturers. The 26th and 27th frets are still a bit hard to get up to just like the original, and it still has that medium sized D shape neck. Apart from the wood, the biggest difference is actually in the switching system. The pot is a CTS pot, and the switch is a high quality 3 way switch. The problem is that the switch can be a real pain to switch on the fly as the floyd's bar tends to get in the way. One interesting thing worth noting is that the rear electronic cavity route is not the typical Horus route but the Dellinger/TAT route. I'm not sure if Itaru did that for tonal purposes or for customization purposes.
The sound on this thing is absolutely killer. It's currently my #2 guitar at the moment. The walnut wood is similar to mahogany, but it has maple overtones going on. It sounds really fat, yet it still cuts through without a problem. I'm not a fan of the stock pickups, so I replaced the neck with a Hot Rails and the bridge with a Custom. The Custom really fattened up the tone, and when tuned to B, it gets that famous melodeath tone that everyone knows and loves.
I'm a HUGE fan of Caparison guitars, and this guitar delivers tone in spades. The walnut body and HGS setup help push Caparison in a new direction with their guitars while still maintaining that familiar feel they're so famous for.