Nice setup there. I have that same engine/pump combo and it works great and is very capable for being so small, light and easy on gas! Mine runs a 2.5" suction nozzle just fine and easily up to the task of supplying enough water for even a larger 'shovel in' highbanker.
If that is a Keene Highbanker you should see if the sluice is compatible with one of their dredge flares.
Here is a short video of my simple setup. Go Pro strapped to my head and those are great little cams, but trying to get things in frame is a bit of a guessing game for me.
I can't tell if you are using 1 hand to film, assuming you are then yeah, hate to say it but in that case, in that spot, you could be well better of with that reducer on there. Trying to film and mine at once is a bit of a chore. Now if you had both hands to mine with then yes without a doubt its holding you back.
You mention plug ups and yes we all get them on occasion. There are things you can do to help minimize them.
1. Keep the suction hose lift as low as possible. This will be harder to do with the highbanker combo units but every bit helps. I'm thinking mine in the video here could and should be even lower.
2. Try to keep the dredge hose as short and straight as you can. Long suction hose is somewhat a luxury at the cost of performance.
I am NOT suggesting to cut your dredge hose any shorter though. If you have 20' of suction hose keep it 20' and try to find another shorter section of hose 10-15'.
I think you would be very happy with the overall improvement a flare attachment would make. Though it would mean making changes to your sluice riffles to get the most from it. I must warn you once you start down this road the more success you have, the longer the road becomes.
Rock jams. Most of mine seem to happen in the nozzle itself. If you fit a valve where your pressure line connects to the nozzle you will find it a useful addition.
