TopspeedLowet

The merge collector if properly sized to the engines horsepower will net gains in horsepower and torque depending on how well the primaries are suited again for the horsepower of the engine. The throat of 3.5" is big for 1000 hp, a 3.25" is closer to appropriate for your engine. I have used Chris Hill at SPD in california. They advertise in your national dragster. Chris has helped me design headers and provided me with working collectors that he and I design based on the engines horsepower and peek RPM. Over a set of dyno headers that look like your existing headers we gained 45 Horsepower on my own engine last year with a merge collector and a step design that had the steps in the correct locations for the engine or at least it sure liked them. Always run a merge collector over a standard 4 to 1 vanilla piece. The entry angles ideally can be 12 degrees but this makes the collector very long and not fit under some cars. They more abrupt the 4 pipes come into the merge cone, the less gain you can expect with the new collector. On your existing collector, the pipes are at 0 degrees. The 12 degree one will have a 12 degree bend in each primary on the collector to point at the throat which makes a nice gradual transition to the throat. The lower photo the degree appears to be between 15 and 25 degrees. The 15 and less desirable 24 degree ones have more abrupt transitions and this hurts the horsepower measurably. In conclusion, Run the merge collector but if you are willing to order one you will benefit more if it is designed for your engine and has the more gentle 12 degree transition in the primaries of the collector.
Food for thought.........
I would have sang in earlier but got burned out on header questions a while back and did not notice who authored the question, Sorry Dave
Bruce