That's why I lower the voltage and the max TDP of the card. Lower the voltage can keep the card run cooler and virtually silent. Limiting the TDP can keep the card away from hitting the Mac Pro' shutdown protection.
A single 7950 can draw up to around 185W, but this figure usually only occur when carrying out stress test with stock settings. It's good the make sure the setup can handle extreme scenario, but I have no plan to use my card in that situation daily.
In my case, I'd test my system with both card set at 1.05V 970/1500MHz and virtually no artificial TDP limit, of course the power draw will be way over 75W from each mini 6pin, but the system is perfectly stable.
For daily ops, I run both card at 0.956V 800/1250MHz. And the graph below is the real world demand when editing / rendering a short 4K video. (The left most 30% is obviously idle, mid 30% is editing, right most 30% is rendering)
I am those kind of person love to push the system to limit. Otherwise, I consider it's a waste of resources.
The naming in the software is wrong. Obviously the mini 6pins are the line that supplying 85W and 76W.
I am totally happy with the 76W, even though it's technically 1W over the limit. I am 100% sure it won't cause any issue since it just happen momentarily.
The 85W may looks quite worse, but in my own experience, the system is really OK to handle 100W continues loading on a single mini 6pin. Even though on a single card system, I often OC the card to allow it to pull close to 100W continuously in Windows for gaming. And now, this 85W again is just momentary loading, no big deal at all. I am quite sure that 75W limit has extreme large safety buffer.
IMO, that 225 (75W x3 ) limit is quite meaningless. So far, I never see a card that really distribute the load perfectly between the 2x 6pin and PCIe slot. e.g. The 7950 Mac Edition card can draw up to around 185W under Furmark. Well within the 225limit. However, if we check the loading carefully, It may only draw 35W from the PCIe slot, another 65W-75W from one of the 6pin, and another 75-85W from the remaining 6pin. This is what the "official" 7950 actually can do (in stock voltage and settings). It can exceed the 75W limit. So, now, that 75W more like a reference for me, I would like my normal ops average loading under that figure, but I won't really treat it as an absolute limit. And I am quite happy to go beyond that 75W under high demand.
In fact, I am very happy with my current setting. When FCPX rendering some 4K video, the average loading on each 6pin source stay between 60-70W, which is a very good utilisation to the system's resources.
N.B. Obviously I am not talking anything about the 980Ti, but I have no intention to hijack this thread, I am just sharing about my experience on GPU power draw. By considering the 980Ti has 250W TDP. It can (and will) draw more than 75W from a single 6pin source. I believe that running the 980Ti with internal power only (without mod) is a hit or miss is because of the uneven power draw distribution, not really because the card really drawing more than 225W. And I must emphasis that I am NOT recommending anyone intentionally go beyond that 75W official limit under any circumstances.
. The Mac Pro's are over-engineered and over-manufactured but going 30 percent over official rating is asking for trouble !