Which model did you install?
No, the Y-pipe swap is NOT an indispensable part, but it will "open" up flow better than what you currently have and supplement your AM exhaust nicely. As I've whistled before on these boards, the OEM Y is horrid!
After the fact, as you now will have to if you decide, it's not an expensive or time consuming install for a reputable exhaust shop. You can grab the Y for right around $30.00 [HERE
] and install should cost you app. $60.00 - $100
So for no more than $130.00 [parts n' labour] you get a MUCH better flowing Y with probably a 30% [or greater] less restriction in exhaust gas flow.
IMHO, absolutely worth it!