Upper Tier Decision (Score 1)

You have posted a good architectural question yet nobody here could truly answer that for you with the general description of your system.

My experience is that Shit Flows Downhill so head higher up the hill and ask a person more closely involved with the customer, contractor, etc..

This might be a good time for an evolutionary prototype where you create a business tier class that allows for more portability. You then create some PL/SQL, T-SQL , (whatever RDBMS you are using) code and test the performance differences. Show the results to the guy higher on the hill and explain to them the benefits of both solutions. You will soon hear what they truly want. This is also good from a job security standpoint because creating this type of presentation and keeping an email trail can always show that you considered all possible scenarios. Somebody comes to you later on and says "what the f*ck bill? We can't ever get this over to MySql now.. What were you thinking?" You just show them your prototype and email trail and watch them scurry off to bitch at somebody else.

My other experiences have shown me that these guys higher on the hill aren't worth a damn unless they have something they can actually see in front of them anyway. :)