Pick a number. It doesn't matter - you're wrong. Bruno Senna was underwhelming, with a tendency to go backwards in races and be out-qualified and out-scored by his team-mate. "Jumping into the car mid-season" might be an excuse if it was Senna's rookie year ... but it wasn't.
"Occasionally" seems to be the key word here. I'd be much more impressed with Senna if he was say, "frequently" faster than Petrov, or "consistently" faster. He was neither.
Wow, so a driver with significantly less experience in the Renault, with KERS, DRS and Pirelli tyres, fails to frequently beat his teammate who is much more experienced in those fields, and he is underperforming?
Brilliant logic.
Jumping into the car mid season means, first season or not, that he is going to be very inexperienced with the car. Petrov got hundreds of kilometers of testing in pre-season. Senna didn't. He drove the car for one day. and even then, only used the hard tyre. Petrov also had more than half a season with the car already. Senna, again, did not.
Now, Bruno Senna, stepped up at Spa, and outqualified Petrov on his debut, and that doesn't convince you in the slightest that he's worth the seat, and not underperforming? No, because you are the Joe Saward of Vitaly Petrov. He did it again in Singapore, and at Brazil. How about now? Nope, Mr. Saward thinks not.
Don't forget that even though Bruno came in mid season, he was still Renault's test driver before and has had quite a bit of testing in the car before getting Heidfeld's seat. So you couldn't chalk it up to not knowing the car.
He drove the 2011 car twice before Spa, in FP1 at Hungary, and for one day in pre-season testing. Yes, that is some experience, but not much, certainly not enough to really get in touch with the car.