Mesa offers this cab in an 8 ohm version, mainly so they can sell you the 8 ohm cab. That guy might not be in the know, but they do. People on this forum have it. In reallity, it is going to be 3-6 dB louder because you gain the extra cone movement (I was never sure if cone area was a 3 or 6 dB gain). The power will be the same coming out of the amp at 4 ohms, no matter what you decide. Do you really need the extra volume? Are you running the cab you have now a high levels? If so, and you want the extra woofer, then you really need to order an 8 ohm extension cab, and a replacement 8 ohm 12" woofer to replace your 4ohm with, then you will be back down to 4 ohms total. Remember that using the combo without the extension cab will bring it down to 8 ohm, and that amp only puts out 150 watts max into 8 ohm, which will make it 3dB quieter than it is now. So you will most likely always need both 12s! This is all in "loose" theory though.
More importantly, how long was the store running your rig at 2 ohm (Assuming they were running the whole stack as a demo)?
Wes