Query performance with date parameters

I have query performance issues when using date parameters in a stored procedure.  The proc takes 15 minutes with hard-coded date values but well over an hour with parameters. I was calling it from SSIS and had trouble passing the variables as datetime2. So I was passing as varchar and handling the conversion in the proc.  Then I took SSIS out of it as i tried to debug. I've been searching for quite some time and I can't get anything to work. This is basically where I've left off.  (SQL2016)

EXEC dbo.GetData @ObsStartDt = '1/1/2017', @ObsEndDt = '12/31/2017'



CREATE PROCEDURE [dbo].[GetData] @ObsStartDt varchar(10) = NULL, @ObsEndDt varchar(10) = NULL
AS

BEGIN TRY  
SET NOCOUNT ON;


declare @StartDT datetime2  
declare @EndDt datetime2

set @StartDT = cast(@ObsStartDt as datetime2)
set @EndDt = cast(@ObsEndDt as datetime2)

select  <stuff)

from HealthPlanDM.MED_CLAIMS_FACT a
join HealthPlanDM.MEMBER_DEMO_DIM b
on a.MEM_DEMO_KEY = b.MEM_DEMO_KEY
join HealthPlanDM.PRODUCT_DIM c
on a.PROD_KEY = c.PROD_KEY      
join HealthPlanDM.DIAGNOSIS_DIM d
on a.DIAG_KEY = d.DIAG_KEY
join HealthPlanDM.PROCEDURE_DIM e
on a.PROC_KEY = e.PROC_KEY
join HealthPlanDM.SERVICING_PROV_DIM f
on a.SERV_PROV_KEY = f.SERV_PROV_KEY
join      HealthPlanDM.POS_DIM g
on a.POS_KEY = g.POS_KEY
join      HealthPlanDM.DRG_DIM h
on a.DRG_KEY = h.DRG_KEY
join HealthPlanDM.TOS_DIMBASE i
on a.TOS_KEY = i.TOS_KEY
join devwork.ACG_PATIENT_EXTRACT1 P                  
on b.MEM_ID = P.patient_id  
left outer join [Provider].NPI_REGISTRYBASE REG
on f.SERV_PROV_NPI = REG.NPI
left outer join DevWork.TAXONOMY_XWALK TAX
on REG.PrimaryTaxonomyCD = TAX.TAXONOMY_CODE
where c.PLAN_CODE <> 'MHACO'  and    
 a.ALL_AMT IS NOT NULL                                                            
and a.DOS between  @StartDT and @EndDt

OPTION (RECOMPILE)

end try