You won't be able to get rid of that much of weight without loosing some functionality, eg carrying luggage. My S weighs about 150kg fully equipped for the desert including 10 litres of fuel. Equally equipped the E of a friend of mine weighs about 5kg less.

And for a trip into the desert (I concluded that from you mentioning extended soft sand sections) I would suggest to put the emphasis on reliabillity and arriving safely (or at all). In the desert it's not about how much a spare part is or from which dealer you get it. It's about reaching your destination with the means you have on your hands. Put on reasonable protection for the machine: case savers, some kind of radiator protection. Repair kits for tubes and some spare tubes including the tools to really do it, spare levers (front brake, clutch, shifter), a clutch cable.

The gains by tuning the S are negligible. I never had a problem to keep the pace with a friend of mine who rides an E. And as you know, that means 49 against 39 ponies, about 25%. The speeds you ride in the desert are much lower then you anticipate. Forget what you saw watching the Paris-Dakar. In Tunisia (short dunes) I used a transmission ratio of 14/47, for the long distances in Libya I switched to 15/47.

The rear subframe of the S and the E are quite different. The one on the E is considerably shorter and can not take any weight behind the seat. Over here the S is homologated for a pillion, the E is not. Would you believe drawings from the repair manuals of the manufacturer?

A drawing of the complete frame of the E:
Ronayers.com Fiche Desktop Motorcycle Suzuki 2003 DR-Z400E FRAME

A drawing of the complete frame of the S:
Ronayers.com Fiche Desktop Motorcycle Suzuki 2003 DR-Z400S FRAME
(Flash Plug-In is necessary to see the drawings)

The plastics of an E can be made fit on an S, but they won't fit well around the area where the rear frame of the E ends. You will have to cut a way for the larger sub frame. I'm cheap so I got me a used set of plastics from an E and fitted them to my S. I don't have my bike to look at it.

The rear mudguard on mine is from a KTM EXC. Fitting it means to cut off a good portion of the upper part. A nice touch is that the angle of the part that holds the rear light has the same angle as the original part so the rear light sits level.

Don't know about the mounting of the exhaust on the E, but maby the drawings above can help.

IMS tank and Unabikers don't go together on one bike. You will have to ditch them if you go for an IMS. There is not enough room for the Unabikers underneath the IMS (I tried). If you want to keep them, go for an Clarke tank.

No, in my experience it doesn't. First I had am IMS tank, then I had a dented radiator. After that I got a Clarke tank and Unabikers. No problems since then. It is possible however to build an additional support for the IMS tank that fixes the two sides to the main frame in front of the cylinder like my friends did. But the IMS tank should not brace itself against the radiators.

Think about it: a broken cooler in the desert you can't fix means someone else transports your bike, your gear and yourself or you leave it where it is. And don't bother to come back to pick it up again instead you're willing to make someone stay with it until you come with the spareparts and the means to repair it or a transport back into civilisation. Food and water for how many days can you spare for that person?

Now my S has about 16.500km on the clock, a third of it comes from riding in Tunisia and Libya.

Pictures from Tunisia are here: Tunesien 2006/2007, Album 1

Pictures from Tunisia and Libya are here: Tunesien und Libyen 2007/2008, Album 1

__________________
Seen whilst travelling - Fernwehstammtisch Karlsruhe - Relief action Togo Togoville e.V.