Mostly for bling. There is some more HP at 5500-6000 RPM but mostly with a more agressive valve train. Either the Gil or the CMI's will provided added HP if run dry. No matter what you do, you are not likely going to gain more than 2 MPH...maybe nothing.
If bling is not a goal, I would sell them.
We used to be able make 500+ HP running dry Gil's with 3.5" ID X 4.0 OD tails. You would also need to do work on the CMI tails to get then to fit your transom cutouts and then you will likely need new transom trim rings if tails have different OD. You will also need the CMI water manifolds and new hoses. It is likely the installation cost will exceed the CMI's initial cost.

With all this, they better look good!

Easyer to change the plugs !

Gils and similar manifold type headers usually had risers that leaked after a while. You don't get that with headers, hence the reason more people use them.

I keep trying to explain to performance boaters what the term "Headers" really means. I do not consider E-Tops real headers, they are really short tube tubular exhaust manifolds.
True headers really should have longer 16- 20" tubes as close to equal length as possible with a true merge collector where this actual header can scavange the exhaust from the engine and provide increased air flow and volumetric efficiency to the engine, not just look kinda like a header that replaces a good exhaust manifold which inside is essentially the same as the E-top. Got It! good, now go buy yourself a real header or keep the Gils you've got!

Best Regards,
Ray @ Raylar

Thanks Guys, think ill pass on them and stay with the gills.

The primary lenght for the E-Top is 15.5", within about .25". If there is no power increase over the manifolds, why was there a weight penalty in APBA production classes when the Gills were replaced by the CMI's on HP500 carb engines?

Seems to me that since the Gils all flow together early in the manifold there is more chance of reversion than on a CMI etop. It was my understanding this was the reasoin Mercruiser went to the CMI's.