The most recent Dell Optiplex 7080 Micro has a PCI slot option now, except it orients the GPU facing down. This might give you about half a slot height of extra space for heatsink/fans...
Have you tried any of the various Thinkpad fan control utilities around such as TPFanControl? It is possible that the Thinkstation uses the same or similar embedded controller, so it may work (such is typically the case in Dell world)