A lot of ad-hoc queries impact on server We have a third party application which generates a lot of ad-hoc statements (our server has 16GB of memory and there are 140000 ad-hoc queries present in the cache).I've enabled 'Optimize for Ad-hoc Workloads' so the query cache doesn't get bloated.
I've asked the supplier to adjust his queries but he doesn't want to change them.

My question is, what impact can a lot of ad-hoc queries have on my server.
My first thought is a lot of compiles.
It can impact the amount of memory available. Each query plan takes a little memory away from SQL Server. 

Also, each time a plan has to be created, SQL Server has to compile it. This means it uses CPU which could be used for other processes. If you have a high CPU contention than that could become a problem.

This query can help you see how large the query plans actually are:

SELECT TOP 1000
    st.text ,
    cp.cacheobjtype ,
    cp.objtype ,
    cp.refcounts ,
    cp.usecounts ,
    cp.size_in_bytes ,
    cp.bucketid ,
    cp.plan_handle
FROM    sys.dm_exec_cached_plans cp
    CROSS APPLY sys.dm_exec_sql_text(cp.plan_handle) st
WHERE   cp.cacheobjtype = 'Compiled Plan'
    AND cp.objtype = 'Prepared'
ORDER BY cp.usecounts DESC;