Ok, just did your experiment.
Here is my results from just the best of 2 laps each. Test car: stock McLaren MP4-12C
GT5: 4:25.218
FM4: 4:26.068
It does seem GT5 has a shorter lap (or FM4 is just longer) I felt like I ran a much faster lap in FM4 though. I was able to feel the limits of the car much more than GT5. With that being said, even though I have full seconds I can improve on either game I feel there is even more to improve on GT5 even though that was the quicker lap. My braking zones in GT5 were too early since I wasn't used to it and couldn't feel the car as much.
On another note. If you ask me, even if the FM4 version is longer than real life I couldn't care less. It's just much more enjoyable to drive when you can actually feel what the car is doing. GT5 felt so bland after playing FM4 for the past week. It was so good to put FM4 back in and get proper car sounds as well.