Interesting, potentially confirms my theory that you need to ramp up the nMP fan manually to deal with the D300 and D700 as both are highly clocked compared to the D500.
Also makes me wonder if they discontinued the quad partly because it came with the D300.
The Quad (and D300) was discontinued because their relative performance against stuff currently on the market (including more than few iMacs) gets beat in more than a few benchmarks. Apple didn't want to give up the price points so they just rotated stuff down one step to next lowest established mark. The configuration at the lowest entry point just disappeared. It is like musical chairs. If at the bottom and there are no more chairs to sit in ... your are out of the contest.
[ Pragmatically it hasn't disappeared completely. There have been a bunch of Black Friday / Cyber Monday / "Please buy something before holiday buying season over" fire sales on the Quad/D300. The inventory is still still parked out there at lots of retailers. Apple just chucked them from their financial ledger. Apple probably wouldn't allow $1999 pricing (e.g, B&H Photo at the moment) on Mac Pro sales if the Quad still had an "official" price. It doesn't so it can sell sub $2000. ]
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Pretty sure that is the snap from Marques review of the iMac Pro. He has a 10 Core model. if trying to maximize single thread performance that is the wrong model.
It is has a Intel W 2155 10 cores , base 3.3GHz. Turbo Max 4.5GHz.
https://ark.intel.com/products/125042/Intel-Xeon--W-2155-Processor-13_75M-Cache-3_30-GHz
The 8 core model should have a W 2145 base 3.70 Turbo Max 4.5GHz.
https://ark.intel.com/products/126707/Intel-Xeon--W-2145-Processor-11M-Cache-3_70-GHz
Probably not a huge difference but the 'back off' from the max is lower. None of these are going to hold absolute max constantly for long periods of time. Mostly likely real sustained max is very slightly lower. But yeah an iMac 2018 update later in the year that got something like an i7 8700 (
https://ark.intel.com/products/126686/Intel-Core-i7-8700-Processor-12M-Cache-up-to-4_60-GHz) that can Turbo max at 4.6GHz ( or an 8700K at 4.7GHz ) are probably going to squeak out single core drag race wins on reality short benchmark runs (not necessarily long sustained for more than several minutes runs ).
If worried about single then base closer to max will probably push the single thread drag race higher.
It wouldn't be surprising for Apple to drop a new Mac Pro with a W 2125 (
https://ark.intel.com/products/126708/Intel-Xeon--W-2125-Processor-8_25M-Cache-4_00-GHz ). 4 core : base 4.00 turbo 4.50 at a entry level $2,500-2999 price to hit those who are looking for LogicX corner case where don't need "big" GPU , integrated screen, and necessarily relatively high core counts, but do need a single PCI-e slot to hold some specialized gear that represents high sunk costs. Leaving the 4 and 6 core Intel W models out of the iMac Pro makes some sense if going to use them as differentiators later in a Mac Pro. Plus, the mainstream iMacs are getting their own 6 core and single thread models (as mentioned above) so less overlap between iMac and iMac Pro too.
[ late update: P.S. I forgot Apple is using the "B" versions of the 2145 and 55 where the base clocks are gimped. That gimping gap is probably holding them back more than noted above on single thread drag racing . And since this is a Mac Pro subforum ... all the more reason they should use a set of non gimped Intel W CPU in a revised Mac Pro. ... if they had courage enough to do that. ]