I don't think it's ridiculous to use the TITAN Black or TITAN V.
They have the highest DP-Performance per Watt on single GPU-Cards according to the wiki-lists.

Would like to have one of them, especially the Black costs sometimes on ebay only around 250-300$.
Sure the V with around 3000$ is somehow ridiculous if you buy it only for BOINC but if you already have it for work and let the card work in the night for you cannot be as bad :P.

TITAN Black:
SP: 5120
DP: 1706
TDP: 250W

TITAN V:
SP: 12288
DP: 6144
TDP: 250W

Radeon 7970 GHz-Edition:
SP: 4096
DP: 1024
TDP: 250W

Radeon RX Vega 64:
SP: 10215
DP: 638
TDP: 295W

https://en.wikipedia.org/wiki/List_of_Nvidia_graphics_processing_units
https://en.wikipedia.org/wiki/List_of_AMD_graphics_processing_units