You've updated your video card driver, Pepe, so that takes care of what I described.
I look at this from the perspective of a production machine in my business, or in a client's business. I can't afford to rely on a machine that's out of warranty, because I can't afford the longer downtimes of such machines and their higher maintenance costs. On top of that, as you've noted, someone has seriously reworked the card. Who knows what they've done to it. But such things may be common for these cards on ebay. These are the things you deal with for an older machine that's out of warranty.
You can flash the vBIOS, but if the new vBIOS you're flashing to is incompatible with the version of the card's ROM, or if something interrupts the flashing process, or if the new vBIOS you're flashing to was mislabeled or doesn't happen to be one of the ones on our list, or is corrupted, then you can end up with a machine that won't boot. And at that point, you won't be able to flash the vBIOS anymore to any other version. So if your applications work adequately without flashing anything, then I'd leave the vBIOS alone.
New high-end 17" laptops are so much faster than the W700, especially with the faster GPUs, especially if you use a RAID0 of SSDs on SATA3, load up with 32GB of RAM and use a RAM disk, use an mSATA SSD for backups, use USB 3.0, eSATA, Thunderbolt, etc. And many new machines give you a display that's at least 100% sRGB (the gamut of the W700's display), and IPS is available, not to mention the fabulous Dreamcolor displays, and 3D is available. And speed/time is money.
So this is the context in which I see all of this. As an emergency backup machine, which is the role of my W700, I'd run it on a cooler, keep the vents clean, and minimize the intensity of thermal cycling (i.e., don't load the GPU any more heavily than necessary). And enable Windows Remote Desktop on the W700, so that you can control the machine if it fails in the way described by Pepe and ElbertR.
And given that today's high-end mobile GPUs can draw 100 watts, and all use lead-free solder (which cracks over time instead of flexing with the expansion/contraction of intense heat), I'd run a notebook cooler on the new high-end machines as well, to minimize temperature swings, just like Hans mentioned. I had to buy a high-end gaming laptop to get high-end performance, and to meet my requirements at a reasonable price. Gamers beat the heck out of their laptops on a daily basis by running the processors full-tilt for hours at a time, and most who I know are now running them on coolers, because they've found out how hard the heat is on their machines.
Nvidia moves to a new fabrication process usually every three generations. This is usually what provides the largest improvement in performance. With the 680M, they moved to a 28nm process. So I don't expect any significant performance-per-watt increase until the third generation after that. So that's when I prefer to buy laptops, and that's why I bought one with the 680M. The 780M only improves performance 10-15%, which is hardly noticeable, and its successor likely won't improve performance significantly without generating more heat. If you don't require the "luggability" of a high-end laptop (I do, for demos), then a new desktop machine will give you more bang per buck.
Jimbo
The ThinkPad W700 Resources Page