It's not legal anywhere that I know of, but even if it was, it's unlikely to do you any good since I doubt you'd have an easy time finding a download for the release version of the game. The best way would probably be to find a used physical copy online on the cheap, actually. You can often find them for just a couple bucks these days, and while they have DRM in the 1.0 version, the very first patch removed it entirely.
Then again, all of this is operating under the assumption that my memories of the game's performance several years ago are accurate. Even if they are, though, the difference in performance between the two versions is likely to be negligible.
EDIT: I feel weirdly responsible since I said that without having ever looked into it very deeply, so I'm going to try to find time soon-ish to reinstall the game and take it through various patches to test the performance of the patch versions relative to each other. I'll be sure to let you know if I find anything significant or noteworthy.
EDIT2: Okay, finished results, benchmarked in the notoriously difficult-to-run Flotsam forest:
Patch 1.1 with Ultra settings (minus Ubersampling):
Average FPS: 35.199
Patch 1.35 with Ultra settings (minus Ubersampling):
Average FPS: 33.16
Patch 2.1 with Ultra settings (minus Ubersampling):
Average FPS: 33.12
Patch 3.4 with Ultra settings (minus Ubersampling):
Average FPS: 30.559
Patch 1.1 with Low settings:
Average FPS: 56.352
Patch 1.35 with Low settings:
Average FPS: 47.38
Patch 2.1 with Low settings:
Average FPS: 48.334
Patch 3.4 with Low settings:
Average FPS: 46.532
There we go—some concrete proof that the earliest versions of the game run best. The Ultra specs haven't had their frame rate change very much from patch to patch, but the Low settings have become less and less useful. It's worth mentioning that the patch 1.1 benchmark is using a different save due to save incompatibility between certain versions, but all other examples were using the same save, and all of them involve running around Flotsam forest (so the slightly different starting locations shouldn't affect the frame rate much).
Post edited June 19, 2014 by 227