Optimizing the UPDATE For MSdistribution_history

One of the major headaches in my replication is the following MS procedure (over 55% usage), updating the distribution history. It currently has one index on columns agent_id, timestamp, runstatus, start_time and time. When I run a select an add some parameters, the query plan is two items: SELECT (2% cost) and Clustered Index Seek (98%). Now, generally, I'd say that's as good as I can get with this, but was curious if any experts here had some additional techniques they've tried that worked.

UPDATE MSdistribution_history  
   SET runstatus = @runstatus,  
   time = @current_time,  
   duration = @duration,  
   comments = @comments,  
   xact_seqno = @xact_seqno,  
   updateable_row = @this_row_updateable,  
   error_id =  
   CASE @error_id  
      WHEN 0 THEN error_id  
      ELSE @error_id  
   END  
WHERE agent_id = @agent_id  
AND timestamp = @lastrow_timestamp  
AND ( runstatus = @runstatus  
OR (@update_existing_row = 1  
AND runstatus in (@idle, @inprogress)  
AND @runstatus in (@idle, @inprogress)) )

Appreciate your help.