I'm redoing the exhaust on my friend's prelude because the current set-up is unacceptable and awful. It consits of the following:
1-Missing Bolts that connect the pipe right after the headers causing the exahust to hang too low and scrape.
2-Universal Cat
3-2.5in piping, no resonator
4-Cheap universal muffler

Here is what I propse to fix:
1-Get replacement bolts for that pipe so that it hangs at the correct height.
2-Leave the cat
3-2in or 2.3/4in piping after cat
4-Apex-i Universal N1 Muffler with optional silencer (For his mom)

Now, my problem is whether I should go 2in or 2.3/4, please advise on which is the better size to retain proper amount of back preassure but still gain HP. Plus my friend wants a really loud exhaust which is why I chose the Apex-i unit. Thanks in advance.

 

Most aftermarket catbacks are 60mm or 2.25" depending if they're US or non-US, I would try to stay with that size. Smaller may cause too much back pressure and put you back around stock and 2.5" or bigger isn't really warranted unless he has some nice mods.