...Fixed radio to fixed radio you probably won't be able to tell most of the time. Talking to the USCG over Rescue 21? Probably not discernible. However, calling a marina when the dockmaster has a handheld and is out walking the docks? It will make a difference that may mean the difference between contact and not...
...Don't kid yourself. The GX-2150 does have a splitter. It's built in to the radio. All the same deficits apply.
I am familiar with most of what you describe, though I do not have nearly as much experience. It's obvious that splitting the antenna output two ways would reduce the strength by 3 dB. As you point out, S/N is just as important, and is why I have my home TV antenna amplifier right at the antenna (so I'm amplifying the purest signal, and not amplifying the noise that my wires pick up).
As a technical matter, the GX2150 does have a splitter somewhere in there, since there are at least 3 radios - the standard VHF radio, and the two separate receivers dedicated to AIS channels 87 and 88. But unlike when you mix and match two devices from two different manufacturers, the GX2150 is designed, tested, warranteed, and reviewed as a single unit, so the complications that you describe are addressed by the manufacturer. So while I don't dispute the theoretical issues that you mention for splitters, as a practical matter I've never heard any complaints about the GX2150 having "all the same deficits." In fact, the reviews that I have seen have been overwhelmingly positive. Can you point me to some test results that show that the GX2150 has difficulties hailing dockmasters at marinas?
As for your suggestion about installing an AIS antenna on the spreader of a small boat, since small boats have short spreaders, the antenna would have to be pretty close to the mast. Wouldn't the presence of the mast so close to the antenna create a blind spot due to shadowing of the signal? Personally, I'd rather have the antenna at the masthead where it gets an unobstructed 360 degree view, and maximum distance to the horizon.