Lowering the bike lowers the center of mass and the moment of inertia. This makes it easier to change the lean of the bike when turning or changing its line. It also helps shorter riders reach the ground. On the other hand, this increased flickability comes at the expense of ground clearance and, possibly, suspension travel. Hard parts can run into each other. Raising has the opposite effect.
2. Safe limits regarding bottoming
The safe limit of lowering is about 1-1/8" or 28mm in the rear of the DL1000 and the DL650. The front limit is about 13/16" or 20mm for both bikes. DL650s with ABS and fork braces are more limited. The Superbrace that reaches a little above the top of the fork legs forces a lowering limit on ABS bikes to about 7mm. Other braces that are even with the tops of the fork legs allow about 10mm. If approaching the limits, test your specific bike to insure safety.
3. Front rear relationships with regard to geometry
As far as geometry is concerned, having the front lower than the rear in relationship to stock decreases rake and trail, making steering quicker, the bike easier to change line but making the bike less stable in a straight line. Lowering the rear more than the front has the opposite effect.
4. Front rear relationships observed
It seems however, lowering the front by 10-15mm with respect to the rear seems to be a preferred setting by many. It has the benefit of making the bike more agile as geometry would predict, but many report the bike is more stable, contrary to geometric prediction. Conjecture is the lower front lowers the angle of attack of the fairing through the air, creating a down force or, at least, negating or abating any lifting force the stock settings provide.
The front can easily be lowered by loosening the triple clamp pinch bolts on one side and tightening them again at the selected position. Then do the same on the other side. It's free and easily modifiable or even reversible. Raising the front is more difficult and requires new/modified hardware.
The rear is normally changed by replacing the dogbone links with different sized links. Longer links lower the rear and shorter links raise it. Some after market shocks also have adjustable lengths. Every 1mm change in dogbone hole spacing makes about a 2.8mm change in rear seat height.
The Science and Black Magic of Suspension Setup