Quote Originally Posted by pundit
According to George Mann you can use standard Boge shock with lowering springs...

"There is a long held myth about so called lowering shocks or shocks designed for lowering springs with reduced travel versus shocks designed for standard or taller springs. The best way to visualize it without physically looking at the shocks side by side is...the only tangible difference is the stroke is shorter in a shock designed expressly for a lowering spring...the throw is shorter. Full jounce position or fully compressed shock position is exactly the same...that is the critical factor for bottoming...wh ere the jounce bumper fully compresses onto the top of the strut collar nut threaded onto the strut housing...that shoulder position is the same for either shock and each shock will identically compress into its cartridge casing. With a standard Boge shock lets say..the throw of the rod internal to the shock is a bit longer..that's all...so the shock is cabable of "extending" farther...at full suspension rebound or when the suspension arms are fully down with a taller spring. With a shorter spring..you don't need a longer throw shock...however..this is the important part...Is putting a standard Boge with longer travel with a lowering spring bad? Absolutely not...you just end up not using the full travel capability of standard shock with a shorter lowering spring... that's all. All the speculation of a standard shock..say Boge..wearing out faster with a lowering spring...is a myth. (From George Mann)"

Full article here... http://www.bmwe34.net/e34main/upgrade/suspensions.htm

Bear in mind that even if there is no mechanical issues using the Boges with lowering springs, a spring with a higher rate (stiffer) will require a shocker with greater dampening. Standard Boges maybe okay with something like the Eibach Prokit springs, but the Sach kit uses Eibach springs with Boge/Sach dampeners which are engineered as a package so I would think that standard Boges with non standard springs would not be an optimally tuned combination. Also if you plan on using your existing Boges they are probably well past their prime and would be even less suitable for the task.
I think that is one of the best answers I've ever recieved here. Thank's.