I haven't had any experience with disco II, but:
Longer travel shockies have a longer closed length. Normally when longer shocks are fitted you either, raise the top mounts, or lower the bump stops. This is to prevent the shockie from bottoming out on full bump travel. The shockie will be destroyed if allowed to bottom out.
Lowered shockie mounts are usually used with a spring lift when stock travel shockies are used (in conjunction with lowered bump stops).
Looking at it some other scenarios:
If you fit springs that provide a 2" lift and leave the shockies and bump stops stock. The axle has 2" more bump travel, but 2" less droop.
If you fit the 2" lift springs, keep the stock shockies, but lower the upper mounts and bump stops 2". You will have the same bump and droop travel as standard.
With 2" spring lift, 2" longer travel shockies, and 2" lowered shockie mounts. You would lower the bump stop 4" to prevent the shockie bottoming out. Your bump travel would be reduced 2", but you would gain 4" droop and the springs would pop out if not retained or dislocation cones are fitted.
The better way IMHO, with 2" spring lift is, 2" longer shockie, raise shockie mounts 2" (difficult at rear unless you have a body lift) and leave the bump stop stock. Then you have 2" more bump travel and the same droop as stock.
Bilsteins are gas charged and those without a remote reservoir, have a floating piston inside the body to separate the gas from the oil. The space taken by the floating piston adds to the closed length.
By comparison, some emulsion shockies may gain more travel for the same closed length.
The gas charge and monotube construction have advantages, but they can be ruined if a rock dents the body (the bodies can be protected - the rears are at most risk).
The above are generalisation and there may be specific features with the Disco II that make some of it not applicable.
