Closing the loop. I purchased a pair of the 40GB VPI with 40GbE on it and swapped cards. ESXi and CentOS immediately saw them as 40 Gbps connections. No updating needed.
Followup question.
The mlx4_en driver in ESX 5.5 stock reports as:
~ # ethtool -i vmnic2
driver: mlx4_en
version: 1.9.7.0 (Dec-03-2012)
firmware-version: 2.30.8000
bus-info: 0000:04:00.0
Is this newer than the one provided on the Mellanox and Vmware links
(Mellanox Technologies: VMware: Firmware - Driver Compatibility Matrix0? It appears to according to version number and date.
If so, should I continue to use or should I downgrade to 1.6.1-2? Can I enable all the features and whatnot with the stock 1.9.7 driver?