Thanks for starting this thread. The mk IV Golf/Jetta diesels respond well to aero mods especially the upper grill block. I have a full (bumper to bumper) belly pan but haven't started a thread because I am a Luddite when it comes to posting photos. I do have some in my albums if anyone would like to move some to this thread.
Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - COcyclist's Album: Belly Pan Install
Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - COcyclist's Album: TDi photos
FWIW I only use the coroplast nose in winter to protect my grill. I stuffed pipe insulation between the grill bars on the upper grill and leave that in place all year round.