On a 2003 BMW M3 (I believe 2002 model is effectively the same, so an
answer for that would be okay too), how much additional head clearance
is gained by deletion of the moonroof? Also, do the manual seats (vs
power) have a greater or lesser magnitude of adjustment, particularly in the
vertical direction?

A source within BMW (individual or web page) would be appreciated;
I've found that dealers have a tendency to make things up.

Hello.

1.2 inches of headroom is gained by deleting the moonroof from the
E46-generation M3 (model years 2001, 2002, 2003) .

M3 coupe (no moonroof).....37.5"
M3 coupe (with moonroof).....36.3"

This was reported on the messageboard of Roadfly.org citing the June
2001 issue of "Roundel" magazine, the official publication of the BMW
Car Club of America:
http://forums.roadfly.org/bmw/forums/e46m3/archives/forum.php?postid=73753&page=4

For more information about Roundel, visit:
http://www.bmwcca.org/Roundel/rdelfset.shtml

With regard to the vertical adjustment of manual vs. power seats, the
consensus on Roadfly.org seems to be that the manual seats will go
approximately 1/2" to 1" or so lower than the power seats.

See this March 30, 2001 message on Roadfly:
"The manual seats provide about an inch more headroom than the power
seats. Just for your reference, the seat is about level with the door
sill on the manual seats, and about an inch above that level on the
power seats."
http://bimmer.roadfly.org/e46m3/messages/archive/msgsy2001w13/63291.html

Also see this January 19, 2001 message on Roadfly:
http://bimmer.roadfly.org/e46m3/messages/archive/msgsy2001w03/42161.html

Also see this December 8, 2000 message on Roadfly:
http://forums.roadfly.org/bmw/forums/e46m3/archives/forum.php?postid=227156&page=1

Also see this April 6, 2001 message on Roadfly:
http://forums.roadfly.org/bmw/forums/e46m3/archives/forum.php?postid=39134&page=7

Thus, it's fairly safe bet that you could gain around 2" of headroom
by skipping both the moonroof and the power seats. Whether you need
the extra room of course depends on your preferences, but one other
factor to consider that I noticed quite a bit of discussion of on the
Roadfly board is that the headroom issue does become especially
important if you ever plan to take your M3 on the track and wear a
helmet.

Another good resource for discussion of the M3 is Bimmerfest.com:
http://www.bimmerfest.com/forums/forumdisplay.php?s=&forumid=14 

search strategy: bmw m3, discussions, headroom, moonroof, seats

I hope this helps. Best of luck with your news wheels.

Superb information and references. Thank you. :)