Dont worry mate I've been having the same issues with HDRI location and actually posted to the MR wish list a solution I'd like to see incorporated. Modo also does real time lighting from HDRI so the results from the illumination channel can be quickly tested.
I've been playing with a work around for correctly positioning the HDRI for the illumination channel, though haven't get it finished yet! Check it out it may give you some ideas, or you may see ways to contribute! Oh hang on you posted to it - duh!
Mate as the last post I always add my skys in later, there are a ton of sky images out there, infact any photo with plenty of sky will work.
Using plastic for car paints can look a bit unrealistic unless you are going for more primary colours that aren't so suited for arch work - the grey, silver, brown inert tones one wants for arch vis are generally metallic and therefore need a coating in the BSDF. Metallics do tend to look a bit flat as the sheen spreads and disperses the light more!
I one day should get back to this project - I can still see a great need for them for Maxwell and any other app they might work with - any that can take weight maps!
http://www.sketchucation.com/forums/scf/viewtopic.php?f=81&t=10730
http://img380.imageshack.us/img380/2652/testad1.jpg
http://img380.imageshack.us/img380/1342/autosavecarqq9.jpg