Need advice: Exhaust setup for supercharged motor

[credditt]

This question pertains to an M44 motor, but the exhaust is so similar that hopefully some forum members here can assist.  

Setup:
Z3 w/ M44
- Downing Atlanta supercharger @ 9 psi
- 3" MAF from S50 6 cylinder
- Larger injectors
- Technique Tuning Stage III software
- Supersprint cat-back (average pipe diameter 2.0 inches)
- E36 M3 clutch w/ Fidanza lightweight flywheel
- Last dyno: 181 rwhp (~100 degree ambient temp, 80% humidity)

My Goal:
- Gain 10+ hp above 5500 rpm
- A small decrease in low end torque is acceptable
- Keep exhaust at or near current Supersprint volume and tone (I dislike really loud exhausts)
- Make exhaust free flowing enough to handle increase boost in the future

My plan:
- Install 4-1 header (Supersprint copy): I know the stock header is very good, but I think the larger primaries and collector will help for a forced induction motor.
- Install resonator in place of catalytic converter (Vibrant Ultra Quiet)
- Install mandrel bent pipe and a larger rear muffler, probably a Magnaflow

I can't decide on the exhaust pipe diameter that I should run.  I'm thinking 2.5", but I know that some guys stay with 2", while others go up to 3". Does anyone have any experience with different exhaust setups on forced induction M42/M44 motors? Any input would be appreciated.

[Danny707]

2.5 will be fine.

3 will be too big IMO.

[marko]

181 whp sounds excellent - keep what you have, as it is a great set up.

bottom end torque is where its at (!) - dont compromise on that for a few extra hp up on top... IMO

good luck either way though...

[doitover]

There have been a lot of comparisons done over the years that show there is little if nothing to be gained by deleting the cat if it isn't plugged.

[bearsbmw]

2.5 is plenty