What dia. exh. pipe to not loose low end power?

Just currious what exhaust pipe diameter is recomended to not loose bottom end power. I know a straight through muffler with stock 2.25 exhaust pipe is popular and I have read that some 2.5" cat backs loose bottom end and some alledgedly gain bottom end. I would like to hear from people that have experience with a straight through muffler and these size pipes. I primarily drive at 3000 RPM or less so bottom end power is important. I previously had an 06 Tundra and the recomendation was to keep the stock pipe and just swap the muffler. The exhaust pipe was
2 3/8". I was thinking of maybe cutting up a factory Tundra exhaust and puting a straight through muffler. Thanks in advance for all opinions and information. My truck is an 03 Tacoma TRD 4x4 with the 3.4L.

 

keep it stock

 

Just to throw this out there, the y-pipe, is 2.25" diameter tubing. The OEM catback is also 2.25" diameter. Considering that two tubes of 2.25" tubing (each side of Y-pipe) enter one 2.25" tube, you can see where the restriction is. The catback should be upgraded to at least 2.5" tubing. Pick you brand by your taste.

Side topic, I run 3" that increases to 3.5" catback.

 

I retract my description. I didn't read what year yours is until after my post.