Thanks.

The points you make are all good ones.

Given their implications, I think for me the operative points are

HEAT: I live in Texas where summertime temps reach 100+ many, many days, and the underhood temps I noticed this year in the S60 were incredible compared to my other vehicles.
As an aside, the valve cover has noticeable sludge (despite my nephew having used synthetic almost the whole 103k miles on the car, assuming the shop actually USED synthetic vs. just charging for it). Heat probably increases deposits.

DOWNFORCE due to smooth airflow: I rarely, if even even touch, 70mph, even on Interstates, so I think it's safe to say that I'd be OK there. I am not quite an old fart yet, but after a nearly 40 year career flying supersonic jet aircraft, I am somehow satisfied to drive near the limit, even with my Backdraft Cobra. I guess I just got it outta' my system. I know, it's sad and pathetic, but hey, it takes all types...

As for a 1% or so fuel economy hit, the T5 I have (sadly) gets such mediocre mileage it probably wouldn't be noticed. I'd complain about the mileage, but everyone says the N.A. cars get 28-32mpg, but the 23mpg I get w/ the T5 is near the top of the expected mileage for the turbo car.

FOD (Foreign Object Damage): Valid. I once ran over an alternator that had fallen off the car ahead of me. Fortunately, it did no damage to my Rabbit Diesel (phew!) but it sure COULD have. The plastic shield is pretty heavy, and could probably prevent a solid blow that could crack an oil pan or transaxle case.

All-in-all, I think the extreme engine heat situation, at least in the summer, is the overarching concern in my case for now.

Thanks for the good considerations,
Bob