Comedy show or not, the Stig tests are relatively serious. Yes, weather varies, but the driver stays (almost) the same. (plus they mark the wet conditions). Their tests are no worse than Autocar's or Evo's. In fact, given their very high profile, they are more likely not to play ball with serial manipulators like Ferrari. They are probably more trustworthy than any other British mag.
I have to agree with Krzys on this point. I can imagine that Ferrari begged Top Gear to run the new press 458 for a lap time against the 12C, whether Ferrari will admit it or not.
SLS Roadster notwithstanding, the lap times have never seemed overly contrived to me, although I admit that conditions always vary and most of the show is fabrication.
In fact, through all of this 12C/458 brou ha ha and studying the testing results, it appears to me that the only publication that makes a real effort for objectivity is Sport Auto. That is not a self-serving comment as a 12C owner. Rather, they are the only ones that use the same driver and repeat the same battery of tests for each car, but also make note of the prevailing environmental conditions and remark on their effect on the test results. Still not perfect, but probably as close as is feasible, save for having every sports car ever made available for testing on the exact same day (and run at the exact same time of day). Sadly, for all of their effort, they are still subject to driving press cars rather than customer cars.