IMCO Powerflow or CMI E top?

Looking too replace the exhaust manifolds on 454 Mag carbed engine. Engine is currently stock but will get rebuilt in the next year or so with some upgrades into the 500 to 600hp range.
Will stay NA. Question is will there be a noticable performance difference between these two styles of exhaust in this horsepower range? Would like to do the exhaust now before the interior
work is done so i can modify the hatch if needed. Leaning towards the IMCO because of the ability to retain the shift mount and bracing the ski pole.

EMI Thunder will do also for what you are after.

My opinion, for what it's worth, is that unless your trying to get every last RPM and MPH from your rig go with the exhaust that causes the least amount of alterations to your boat and is therefore the most cost effective while still gaining a performance advantage over the stock Merc style cast iron manifolds. There is a performance difference but it's not that much. Noticeable or not depends a lot on your set up and how you drive your boat. Just my .02. Good luck with whichever option you chose.

CMI sport tubes

I had a Nordic Viking and a Nordic Rage. Both had thru transom exhaust. I installed a aftermarket exhaust and saw no gain at all on either. The Rage was a MPI motor. I did a few other mods on the MPI motor and had the ECU reflashed, Saw some gains then. Unless you are doing other mods, don't expect much from exhaust on a stock motor.

Both will wake up your stocker if you tune for it, but you've got other restrictions.
You wont get much more HP without other changes and the manifold offers the most for the least.

the CMI's are great and offer the most absolute potential but without a top end kit you aren't going to see their potential.

The Imcos are a great manifold, and can move exhaust very well and offer much much longer lifespan.

UD