-- existing
DECLARE @StatusOp as VARCHAR(20)
DECLARE @Status as VARCHAR(20)
SET @StatusOp = ' '
SET @Status = 'UNET'
SELECT ...
   FROM mxDocument 
    where (DATEADD(dd,DATEDIFF(dd,0,dtOrigRecv),0) 
      BETWEEN '01/15/2008' AND '01/20/2008')
    AND sProcessProject LIKE 'GRP%' 
    AND Case @StatusOp 
        when ' ' then 1
        When 'eq' then 
            case when sCustomerSys like (@Status) then 1 else 0 end
        end = 1						
....

-- suggestion
DECLARE @StatusOp as VARCHAR(20)
DECLARE @Status as VARCHAR(20)
SET @StatusOp = ' '
SET @Status = 'UNET'

IF @StatusOp = 'eq' SET @Status = '%' 
-- now @Status will always have the search value

SELECT ...
FROM mxDocument 
where 
   dtOrigRecv >= '01/15/2008' 
   and dtOrigRecv < dateadd(day,1,'01/20/2008')
     -- adjust end date of range so any index on dtOrigRecv can be used
AND sProcessProject LIKE 'GRP%' 
AND sCustomerSys like @Status
...