Ah yes - "Class III" I think it is. Twins with a max gross weight below 6,000 pounds can use them. I was remembering, I think it was
@Fearless Tower who couldn't install one in a Beech 18??
But, the GI 275 can be installed in larger/faster aircraft, even Citations and such.
The Bendix/King KI-300 is nothing more than a Sandia 340A with a Bendix-King sticker on it. It's a rebadging/rebranding of the exact same product, which is pretty much King's entire game these days.
So yeah, the same guy wrote the software. And the same guy(s) designed the hardware, too...
Two things, though:
1) The G5 HSI doesn't even come close to using the whole screen. Physically, the GI 275 HSI may be a larger presentation than the G5 HSI. We'll have to get someone to measure.
2) The GI 275 has a much brighter and higher resolution (ie, more modern) screen. My eyes are getting to be borderline and I have no trouble at all reading the 275.
Legally, I think you could do it. I've read the install manuals and I don't think anything would prohibit that.
Technically, you could also do it, but it'd not be as nicely integrated as just having a pair of 275s in that you'll need to make heading/course/baro adjustments in two places, whereas a pair of G5s and a pair of GI 275s will communicate with each other so that you only have to make the adjustment once.