You didn't say, or at least I didn't see, in the article where you were doing this tuning, which environment. Because you didn't say, people might not know, that running DROPCLEANBUFFERS and FREEPROCCACHE are extremely dangerous operations to do to your production system. You've just flushed the cache of all the data and query plans and every application running on the system takes a hit as that data is read from disk instead of memory and the queries all have to recompile (taking longer, as you noted).
That's correct. If you want to clear cache for perticular statement or procedure then
DBCC FREEPROCCACHE [ ( { plan_handle | sql_handle } ]
For example:
select * from table1
GO
SELECT plan_handle, st.text
FROM sys.dm_exec_cached_plans
CROSS APPLY sys.dm_exec_sql_text(plan_handle) AS st
WHERE text LIKE N'select * from table1%';
GO
DBCC FREEPROCCACHE (0x06000800E6057F1EB840340B000000000000000000000000);
GO
Thanks