Brodie
to answer the question in the topic title:
"Do Mac's still have any solid graphical advantage over PC's?"
The answer is NO. And for people doing pretty heavy CG stuff they're actually worse (don't want to start a flame war so I'll explain)
The main problem here is drivers and performance.
First drivers: mac doesn't support nothing above opengl 2.3 (but they're getting upgrade to 3 in a short time, i think...), and has you know opengl had became famous because it used a lot for CG software. Now today you still see examples of this: Mari can't be released to mac, and autocad for mac is receive a lot of complaints because is simply too slow compared to windows version (but don't take this too serious... is autodesk afterall ).
Performance: plain simple it's the same...if the hardware is the same and the software is the same you'll notice no difference (this from personal experience). Now the problem is Companies and Freelancers in this business normally update/buy new hardware about 2 in 2 years (more or less of course) and from budget point of view when you ask yoursef "i need a computer that can run Cinema 4d (this because runs in pc/mac, but replace by any 3d package) photoshop, vray, after effects etc. and I'm going to spend 1500$ in each pc (that probably will be replace in 2 years) what will allow me to work, render, do faster and bigger? a i3 3.0 4Gg ram Geforce 310 mac or a i7 12Gg ram Geforce 470 pc?".
Think like this if mac was better why does Blur (the reference right now in cinematic for games) just use dell workstations? Hell, the irony of irony's; Steve jobs who was for a long time CEO and it's right know the biggest single shareholder at PIXAR, and, well, they don't even use iOS...
Now for me mac took a turn in the wrong direction in this market. I've worked professionally with both at the same time in brand new ones, but it's simply not the same years ago with the IBM Processors on it (anyone remembers? ) and better screens (for the ones who don't know the mac screens we're famous in digital work at the beginning because they normaly had a more colours and had 15% more gamma if i remember correctly). Now we've got the same processors that we have in pc but more expensive and if you need a good or professional screen you don't buy mac you'll probably buy Lacie or something...
A friend of mine, that right now it's partner in company that does multimedia works for tv, web, etc., says that he remember when mac was the only one that opened big files (well he had to left it overnight but it was open in the morning lolol) and the other PCs would simply die trying in the first minutes. He said that for heavy work a mac worksation was the beast backt then. Well, some years have passed and there's not a single Mac in that company, simply because he can't afford to pay more for less especially when he's always on a tight deadline.
Of course if mac hadn't took that turn, they were probably much bigger in CG but certainly not as big as right now in the mainstream (so yeah i understand they're move and i would probly did the same)
BTW i've done work for advertising agencies/architects/tvs etc. (i'm finishing one to deliver on monday right now) and some use mac but normally more just for vector graphical work (illustrator mainly and that's not even 64bits), the big stuff 3D illustrations and Photoshop illustrations are outsourced either to freelancers or CG companies, if they don't have the man and machine power to do it. But things can change as renting render farms get cheaper or when every software move to the cloud (hope not, but that's a different topic )