The Dell spec sheet is correct in so far as this is not a validated thermal configuration. I can confirm that the GPU power headers are on the risers, and that there is a enough room for a double-wide and single wide card. The issue is airflow. In the validated configurations, the GPU kit has lower heat-sinks to allow airflow to the GPUs, and a few other air flow modifications (see below). GPU configurations are also limited to 120W CPUs. Also if you populate all 12 bays with HDDs, you may overload the cooling needed by the GPUs. There are also power considerations if you are using a 1100W PSU and all drives are populated and you have high wattage CPUs and lots of RAM and want to use 300W GPUs.
Some references here:
Dear Sir or Madam,I have bought the Poweredge R730 w/ Single E5-2640 V4 (10C/20T) with 750W 80+ Platinum PSU config and perform well. Now I wish to add the GPU card for extended processing per...
www.dell.com
and from the R730 manual - Page 22
Internal GPU support
The R730 supports two 300W, full-length, double-wide GPUs or up to four 150W, full-length,
single-wide GPUs. Each GPU can support up to 6GB of dedicated GDDR5 memory. Active cooled
GPU cards not supported. The GPUs are installed on the PCIe x16 3.0 interfaces available on riser 2
and GPU-optional riser 3. A system must have the optional riser 3 with a single x16 slot to support
two double-wide GPUs. A standard riser 3 is required to support 3 or 4 single-wide GPUs.
Because GPUs demand high power, each GPU has up to two power connectors for power delivery.
The GPU enablement kit is required for internal GPU installation. The kit contains the power cables
and other items to enable GPU support on the R730 chassis.
Internal CPU cooling restriction
Due to the high power consumption of GPUs, there is an ambient temperature restriction of 30oC
maximum system inlet temperature to ensure adequate system cooling the R730 has one or more
GPUs installed. Note that this temperature is less than the standard environmental specification of
35oC.
Other GPU restrictions
The following GPU restrictions for the R730 are enforced by the order validator:
Requires two processors
Processors must be 120W or less
Maximum of two double-wide GPUs (since they take up two slots)
Maximum of four single-wide GPUs
All GPUs must be same type and model
GPUs require a redundant 1100W power supply and GPU enablement kit
Two double-wide GPUs require the optional riser 3 with a single x16 slot
Four single-wide GPUs cannot occupy optional riser 3 with a single x16 slot
Must have 1U heatsinks and solid PCIe blanks
Tape backup not supported
For a further hint of this, check out the R7525 thermal restrictions matrix here: (Yes, this is an EPYC system, but similar form factor and nicely laid out)
As you can see, the 12 and 24 drive versions don't support GPUs for thermal reasons.
So, all that being said - will you be able to run a K80 in an R730XD? It should work. You'll at minimum need the power cable - Dell Part Number N08NH, but there is only room for 1.
Now should you? I cannot answer that. If you need a guaranteed validated setup, this isn't it. But it should work. You may have to manually run your fans higher (can set this in IDRAC) and I would strongly suggest using the lowest wattage CPUs you can stomach. I would also leave the top drive bays empty, using only the middle and bottom rows.
Finally a disclaimer. This is an unsupported configuration. None of the above is professional advice. You may run into unexpected issues including the ability to cool your GPU, and this post may contain factual errors or omissions.