It's not a case of "doing better". At the end of the day, topping the timing sheets in practice counts for very little. It's about understanding how the car behaves under condition similar to racing. Having read up a little bit, it seems that Renault spent their first day running heavy fuel loads and testing out their brakes. It wouldn't surprise me if Kubica was braking a little heavier and a little earlier than he normally would so that the team could collect data.
Kubica has a reputation for being difficult to please (or, depending on who you talk to, a bit of a whiner). If he's in a car that is going to be two and a half seconds off the pace all season, there's no way that he'd be pleased at all. Quite the opposite, really. So when he says Renault have their own testing program, he's surely telling the truth. The fact that there were six other teams there doing testing of their own is just after the fact.