Whattaya mean by 90? It's not x-shaped like in your sketch?

 

It's x shaped but it's a perfect x. Like with 4 90degree angles lol

 

 

Ok, like a perfect cross intersection? Then it wouldn't matter. Your first drawing was made to deceive!

 

Yup lol

 

What diameter is your main pipe now? You mentioned going dual 2.5". That's way overkill for a V6, even your deadly 3.4! Do you know how to figure the area of a circle (pi x r2) to do size comparisons? Hint: two 2.5" pipes are equivalent to one 3.5" pipe. Go with too big of a pipe, and your low end torque can suffer.

 

I thought the x pipe would create enough back pressure to compromise for the larger pipe. I've seen tons of v6 cars with 2.5 duals

 

An x pipe isn't going to create backpressure, unless you do something like stuff your neighbors cat inside it!
A couple rules of thumb:
Exhaust pipes want to flow in the range of 2.2 cfm per engine h.p.
Straight pipes flow +/- 115 cfm per sq. in of area.
Area = pi (3.142) x r2. Radius is half the pipes diameter (in case you napped a lot in math class).
So, using one 2.5" pipe: 3.142 x r2(1.25x1.25) = 4.9" of area in a 2.5" pipe.
Even though you're not there yet, let's use a round number of 200 h.p.
Your exhaust then wants to flow 440 cfm (2.2 cfm x 200)
A 2.5" straight pipe will flow 563.5 cfm (4.9" x 115 cfm)
Then you have to factor in pipe restriction down the line; total length of pipe, number of bends, cat converter, mufflers. I'm sure there's a formula for that somewhere, but i don't care to get quite that anel.
So sure you can run dual 2.5" pipes like your pals are doing, but they're thinking that bigger is better. That's not always the case, and in your case, will affect your low end torque.

 

I guess we'll see how it turns out lol.

 

Looking forward to your future thread: "Just installed 2.5" duals, now my car feels doggier off the line"!