E39540iV8 - Sounds so simple when you put it like that doesn't it!?! here's some aspects you've not taken in to account. You will find owners of these cars who want an aftermarket unit to look and function exactly like the original, so they'll want full OBC functionality, they'll want it to 'plug n play' with cars that do have DSP fitted as standard. There'd be others who will then perhaps don't mind not having those but want all the usual features of a modern aftermarket unit with the oem looks but also to then 'connect' to their oem CD changer or oem Mic connection etc - also with some E39's with their electronics in the boot/trunk, and some with power etc at the dash area, it's extra costs to cater to the various configurations of oem E39 audio out there. Perhaps a big major factor is the price of such a unit at retail and I'd say perhaps around the $1200-1350 USD level would be about right. Remember also, the premium brands will also test and certify their products, use high grade electronics and ensure the high levels of performance which makes them premium quality. The E39 has been out of production for almost 10 years now, and even when it was in production, demand for aftermarket electronics etc was virtually nil. This is why nothing was ever developed to upgrade cars with the exception really of the iPod interfaces when the iPod suddenly exploded in popularity. None of the aftermarket premium brands will develop a product that's been out of production for a almost decade and also for a very tiny segment of the ownership of these vehicles who perhaps would drop $1200+ USD on a head unit. Keep in mind also, that you'd also need to upgrade the speakers and many owners are also not interested in sound quality increases either! - The Chinese cheaply made units out there haven't exactly sold in big numbers for the E39, due to their low, generic quality components and the high chance of issues in their operation. So it goes back to my initial response - if you want better performance, sound quality and a well made product, then go with a premium brand choice with the 20% loss of oem looks in the dash. If you *must* have oem'ish looks then go for one of the cheaply made chinese units at the risk of having issues etc.
Bigboy740iL - Nakamichi just so happen to be one of the high end brands for audio equipment. Although mainly home audio they do have car products which also have the same ultra high levels of engineering in all aspects of their products. Makes me chuckle, but if you are under the impression Nakamichi are 'crap' what do you is a great quality brand or unit?! Some of the Nakamichi units do indeed look really good in some BMW's, but not so in E39's and other BMW's from the mid 90's onwards.
Either way, retrofitting the oem widescreen monitor and GPS system etc, is extremely expensive and the modules are now at least a decade old etc. For those who *must* have oem only, it's currently the only choice, but in real terms offers very little overall value for money.