Quick exhaust Question about FD
 

I am wondering about some info for my 1994 RX-7. The car is in very good condition and I would like to keep a relatively stock setup.

Here is my question, I would like to put an exhaust on my car. Now I (think I) know about changing piping and removing cats all should require reprogramming of the main ECU (please correct me if I am wrong).

But I don't really want to touch any of this too much. Is there some measure of flow rate that I can compare an aftermarket exhaust to the stock one.

Or if it is only the muffler that is being changed would it really matter. I just figure that if you don't change the air inflow, or outflow, there shouldn't be a need to remap the computer.
2nd part: What do you guys think of a remus exhaust.

Also, the same question I have for a blow off valve. Do these effect the ecu in any way or just turbo relief. These are pretty much the only two mods I would like to perform.