Dopo ore di duro lavoro sono riuscito a trovare un sistema ottimale:

codice:
update 
tbl_tmp_conversion c ,
(select tc.idKeyModel, 
tc.Curr, 
sum(tc.Sep_StdY) as TarReference, 
sum(tc.Jan_CY) as TarActual
from kmsexport.tbl_tmp_conversion c, 
kmsexport.tbl_causalscategories cc, 
tbl_kmstargetcosts tc
where tc.RowType = 'D' and 
tc.Conditions = 'TARGET 2011' and 
tc.Plant = 'All Plants' and 
tc.CostType = 'TVMC' and
tc.idKeyModel = c.IdKeyModel and
tc.Year = '2011' and
cc.IDCausalCategory = tc.IdCasual
group by tc.idKeyModel) as i
set c.CurrTarget = i.Curr,
    c.TarReference = i.TarReference,
    c.TarActual = i.TarActual
where c.idKeyModel =i.idKeyModel;
RISOLTO!