Seas Lotus and Focal Utopia tn51 tweeters... a brutally revealing combination.
The Lotus reference is based off the original Seas Excel w17 magnesium cone driver. It has a 14mm voice coil and a 6mm top plate, and has noticeably less excursion than the latest Excel drivers. There is no ventilation under the spider.
However, it does come in a 4 ohm voice coil configuration and all the parts are treated to withstand the rigors of a car environment. The speaker wiring posts are also very nice screw down types, and not the usual flimsy spade connectors. Another nice thing is that the t/s parameters are optimized to yield a flat response in most car doors. This gives you that punchy, snappy kickbass without any undue bloat or heaviness.
The extremely stiff and straight profile magnesium cone, low loss surround, and thick copper rings in the motor give this driver unparalleled detail and realism, surpassing many other high end drivers and the absolute best that I've ever heard. And in a typical car door, the upper end breakup is not even an issue so far off-axis and recessed behind a door panel. Even so, I'd generally limit this driver to 2khz to minimize distortion and any coloration.
As for the Focal TN-51, this is a pure titanium inverted dome driver with a phase plug located over the center of the tweeter to help keep up the top end off-axis.
The motor and rear chamber is pretty terrible for a "high end" tweeter, not to mention the top end is a bit rough. I also found a somewhat large peak near the tweeter's low end... probably due to the weak motor and high q of the driver. However, when all this is accounted for and corrected, there is something special about this tweeter. It has a clarity that is hard to beat, although the top end rolls off quite rapidly leaving a bit of a "closed-in" sound, what is there is astonishgly crystal clear. Also, be sure to cross this one over high 4-5khz, as it very audibly strains when driven too hard or too low. And expect a bit of a "metallic" coloration, although it's not unpleasant and I believe contributes to the overall clarity of the driver.