Wow... I have never seen a negative word said about a nosecone. They are designed to improve prop bite and reduce blowout by reducing turbulance, improve efficiency (speed) with better hydrodynamics, and allow trimmed up power through corners. On every application I have seen, it was always a very noticeable improvement for one reason or another. Based on the design itself and physics in general I can't imagine any drawback... If it works at 70+, 80+, 90+, how can it not work at 60+? Just curious, I really can't imagine a negative influence a nosecone could have...?

Increased surface area ( drag) at slower speeds ( less than 80)

The 90' and up bravo gearcase was reported to be good
up to 95 mph before blowout.

I agree with this. I ran a year with them on my scarab panther and last year without. I lost 3mph without them so they are going back on. I can't envision them doing anything but good due to the sharper lead into the water. You just may need to try some different props due to more/less bow lift and different slip number to get it optimal. If you have it on I'd leave it on and work on the setup.

Lots of people have lost MPH adding a nose cone. I lost 4 MPH adding one to my Hammer running a 25 pitch labbed Mirage prop and a 468 allegedly putting out about 500 HP. It ran 78 without the nose cone. I always heard that over 80 it would probably help. I thought I was only 2 MPH off and wanted bragging rights of 80. This is based on GPS rather than the happy meter. I couldn't get the bow lift I was able to previously. Result was 4 MPH lost. It really looked cool out of the water on the trailer, but I couldn't give up the 4 MPH. I am confident nothing is being gained and are likely MPH is being lost.

Yes, that's the marketing hype behind them. It all sounds good doesn't it? Of course so does the marketing hype behind Fuel Line Magnets, Tornado Intake Air Scramblers, Fish Carburetors, liberal politics, etc, etc.

Did you try any other props to see if you could get the speed back and then more? To make a change such as a nosecone and expect to not have to adjust other setup parameters is asking for a lot in a boat. Setting up a vee bottom boat for max speed is a bit more complicated than this. You need to understand the variables and how each effect the others.