You should be able to find what you're looking for using Get-NaVol.
PS C:\Users\SBeam> $vol = Get-NaVol vol4
PS C:\Users\SBeam> $vol.Sis.SisInfo | fl *
CompressSaved :
DedupSaved :
LastOperationBegin : Tue May 1 16:00:00 EDT 2012
LastOperationEnd : Tue May 1 16:00:01 EDT 2012
LastOperationError :
LastOperationSize : 74756096
PercentCompressSaved :
PercentDedupSaved :
PercentTotalSaved :
PercentageSaved : 38
Progress : idle for 1242:25:08
Schedule : auto
SizeSaved : 4294975488
SizeShared : 2147491840
State : enabled
Status : idle
TotalSaved :
Type : regular
Hope that helps!
-Steven