select top(1) with ties * from T2 where Data < @Data1 order by ROW_NUMBER() over(partition by Id1 order by Data desc) asc

Select * 
   from T2 T 
   JOIN (Select Id1, Max(Id2) as Id2 
           from T2 A 
                JOIN ( Select Id1, Max(Data) as Data from T2 Where Data < @Data1 Group by id1) B On A.Id1 = B.Id1 and A.Data = B.Data)
           group by A.Id2 ) M On T.Id1 = M.Id1 and T.Id2 = M.Id2

-- эквивалент
Select m.* 
   from ( select distinct Id1, Id2 from T2 ) as T 
   cross apply ( select top(1) * from T2 Where Data < @Data1 and Id1 = T.Id1 and Id2 = T.Id2 order by Data desc, Id2 desc ) as m

-- из которого видно, насколько идиотичен исходный запрос (обработка ВСЕЙ таблицы, когда реально надо только часть )
-- для ускорения 
-- select distinct Id1, Id2 from T2 
-- можно материализовать (Indexed View)

Select m.* 
   from T1 as T 
   cross apply ( select top(1) * from T2 Where Data < @Data1 and Id1 = T.Id1 order by Data desc, Id2 desc ) as m