Its interesting, as I've actually been thinking about this very topic. A coworker asked about what causes a difference in lap times when the cars are the same model.
The "stock M3" time of1:42.9 vs. my best 1:48.7ish semi-modded, has many factors. The packaging of the car also could be a factor, not sure if its a stripped car with no options other than DCT/comp pkg or are they a similar fully loaded model as well as various traction control settings. I know some of the best times for a car were done during the various magazine testing, the articles even state that some makes have engineers prepping the car. We don't usually have that level of expertise assisting us.
Other factors, in the case of some the more common mods (BBK & tires) don't play as high of a role during their testing/drives IMHO. Its usually a few hot laps, assuming what we see on the various TV shows are true (5th/Top Gear). I have BBK and a square set up yet (Mich PSS instead of PS2). My BBK was due to OEM pad/rotor wear, the square setup was s longevity as well.
I do time just as a measure various lines, tire pressures etc. Overall it boils down to driver skill/experience. I think in the end as long as you work towards bettering your skill and modding what is necessary to keep that possible, you're fine. I personally plan to stay away from R-compounds, engine power upgrades and maybe even suspension changes until I can get closer to that 1:42.9 number. I've been contemplating some of the race schools and perhaps working on my own skills.
2011 M3 Dakar Yellow-Black Leather-Comp Pkg-DCT-Akra Evo-AP BBK Front-VOLK RE30/BBS GT4 18x10" Square
1995 M3 Boston Green - Chipped - Ground Control Coilovers - APEX ARC8
Past: 2005 Porsche Boxster S-Silver-6spd-Fabspeed Exhaust