Germany's Auto Motor und Sport reports that the first whiff of the Briton's worry emerged last week, when the MP4-27 was tested over four days at Jerez.
"When I saw that all the other cars are high at the front, it did make me wonder if we had done the right thing," the 2008 world champion, referring to the 'step' nose trend that McLaren has sidestepped, is quoted as saying.
Hamilton was quicker than his teammate Jenson Button at Jerez, with his 1.19 laptime set on Thursday close to Sebastian Vettel's Red Bull pace on the same day.
The previous day, however, Vettel's teammate Mark Webber had been the quickest of the 2012 car runners in the RB8.
And when referring to his 1.19, Hamilton admitted that the lap was set while McLaren was being "aggressive", implying that he was light on fuel.
If so, the MP4-27 is at least a full second off Lotus' Jerez pace.
There are more clues: asked about the change in blown diffuser rules, Briton Hamilton said the MP4-27 is "difficult to control in the fast corners".
That contrasts with Vettel's comment that, "In the fast corners I don't feel very much difference to before (in 2011)".
Moreover, Spain's Marca sports daily claims Hamilton confided to his former McLaren teammate Heikki Kovalainen at Jerez that he is "disappointed" with his new car.
Over the coming weeks we'll get a better indication of where everyone is
On the record, he said: "I feel quite positive about where the car is at the moment and I'm confident that the guys can push it forward.
"You've got to remember that everyone is just focusing on their own programmes. It's not a contest for who's the fastest at the moment," Hamilton insisted.
"Over the coming weeks we'll get a better indication of where everyone is."