differences between early 99 and late:
1.)injectors early AB 120cc and late AD 140cc
2.)turbo wheel early 88mm late 80mm
3.)intake manifold thingys ealry 2.75" late 3"
4.)Hpop early 15 degree late 17degree
thats all that comes to mind for now
The late is better if your plans are to just add a chip. All out ultimate performance the injectors, turbo, intakes, and hpop will be changed anyway so it makes no difference.
10k mod might make a difference but not for sure. With my old 2 year old ts programs it made a difference but with my new programs no difference.
AIC heres a do it yourself link. Its not too difficult, give it a try.
http://www.thedieselstop.com/contents/getitems.php3?1999-2004%20%2410%2F%2415%20AIC
I think i answered all your questions.-popcorn
thomas